Ragel State Machine Compiler
User Guide

byAdrian Thurston

License
Ragel version 6.10, March 2017
Copyright c 2003-2007 Adrian Thurston
This document is part of Ragel, and as such, this document is released under the terms of the
GNU General Public License as published by the Free Software Foundation; either version 2 of theLicense, or (at your option) any later version.
Ragel is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; withouteven the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with Ragel; if not, write
to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USAi

Contents
1 Introduction
1.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.4 Related Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.5 Development Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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2 Constructing State Machines
2.1 Ragel State Machine Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.1 Naming Ragel Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.2 Machine Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.3 Machine Instantiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.4 Including Ragel Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.5 Importing Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Lexical Analysis of a Ragel Block . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Basic Machines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4 Operator Precedence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5 Regular Language Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.1 Union . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.2 Intersection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.3 Difference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.4 Strong Difference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.5 Concatenation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.6 Kleene Star . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.7 One Or More Repetition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.8 Optional . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.9 Repetition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.10 Negation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.11 Character-Level Negation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6 State Machine Minimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.7 Visualization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3 User Actions
3.1 Embedding Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.1 Entering Action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.2 Finishing Action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.3 All Transition Action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3.1.4 Leaving Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
State Action Embedding Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1 To-State and From-State Actions . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.2 EOF Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.3 Handling Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Action Ordering and Duplicates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Values and Statements Available in Code Blocks . . . . . . . . . . . . . . . . . . . .

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4 Controlling Nondeterminism
4.1 Priorities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Guarded Operators that Encapsulate Priorities . . . . . . . . . . . . . . . . . . . . .
4.2.1 Entry-Guarded Concatenation . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.2 Finish-Guarded Concatenation . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.3 Left-Guarded Concatenation . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.4 Longest-Match Kleene Star . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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5 Interface to Host Program
5.1 Variables Used by Ragel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Alphtype Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3 Getkey Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4 Access Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5 Variable Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6 Pre-Push Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.7 Post-Pop Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.8 Write Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.8.1 Write Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.8.2 Write Start, First Final and Error . . . . . . . . . . . . . . . . . . . . . . . .
5.8.3 Write Init . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.8.4 Write Exec . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.8.5 Write Exports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.9 Maintaining Pointers to Input Data . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.10 Specifying the Host Language . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.11 Choosing a Generated Code Style . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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6 Beyond the Basic Model
6.1 Parser Modularization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2 Referencing Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.3 Scanners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4 State Charts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4.1 Join . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4.2 Label . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4.3 Epsilon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4.4 Simplifying State Charts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4.5 Dropping Down One Level of Abstraction . . . . . . . . . . . . . . . . . . . .
6.5 Semantic Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.6 Implementing Lookahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.7 Parsing Recursive Language Structures . . . . . . . . . . . . . . . . . . . . . . . . . .

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3.2

3.3
3.4

Chapter 1

Introduction
1.1

Abstract

Regular expressions are used heavily in practice for the purpose of specifying parsers. They arenormally used as black boxes linked together with program logic. User actions are executed in
between invocations of the regular expression engine. Adding actions before a pattern terminates
requires patterns to be broken and pasted back together with program logic. The more user actions
are needed, the less the advantages of regular expressions are seen.
Ragel is a software development tool that allows user actions to be embedded into the transitions of a regular expression’s corresponding state machine, eliminating the need to switch fromthe regular expression engine and user code execution environment and back again. As a result,expressions can be maximally continuous. One is free to specify an entire parser using a single regular expression. The single-expression model affords concise and elegant descriptions of languages
and the generation of very simple, fast and robust code. Ragel compiles executable finite statemachines from a high level regular language notation. Ragel targets C, C++, Objective-C, D, Go,Java and Ruby.
In addition to building state machines from regular expressions, Ragel allows the programmer
to directly specify state machines with state charts. These two notations may be freely combined.
There are also facilities for controlling nondeterminism in the resulting machines and building
scanners using patterns that themselves have embedded actions. Ragel can produce code that issmall and runs very fast. Ragel can handle integer-sized alphabets and can compile very large statemachines.

1.2

Motivation

When a programmer is faced with the task of producing a parser for a context-free language
there are many tools to choose from. It is quite common to generate useful and efficient parsers
for programming languages from a formal grammar. It is also quite common for programmers
to avoid such tools when making parsers for simple computer languages, such as file formats andcommunication protocols. Such languages are often regular and tools for processing the context-freelanguages are viewed as too heavyweight for the purpose of parsing regular languages. The extra
run-time effort required for supporting the recursive nature of context-free languages is wasted.
When we turn to the regular expression-based parsing tools, such as Lex, Re2C, and scripting
languages such as Sed, Awk and Perl we find that they are split into two levels: a regular expression
matching engine and some kind of program logic for linking patterns together. For example, a Lex
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CHAPTER 1. INTRODUCTION

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program is composed of sets of regular expressions. The implied program logic repeatedly attempts
to match a pattern in the current set. When a match is found the associated user code executed. It
requires the user to consider a language as a sequence of independent tokens. Scripting languages
and regular expression libraries allow one to link patterns together using arbitrary program code.
This is very flexible and powerful, however we can be more concise and clear if we avoid gluing
together regular expressions with if statements and while loops.
This model of execution, where the runtime alternates between regular expression matching
and user code exectution places restrictions on when action code may be executed. Since action
code can only be associated with complete patterns, any action code that must be executed before
an entire pattern is matched requires that the pattern be broken into smaller units. Instead ofbeing forced to disrupt the regular expression syntax and write smaller expressions, it is desirable
to retain a single expression and embed code for performing actions directly into the transitions
that move over the characters. After all, capable programmers are astutely aware of the machinery
underlying their programs, so why not provide them with access to that machinery? To achieve
this we require an action execution model for associating code with the sub-expressions of a regularexpression in a way that does not disrupt its syntax.
The primary goal of Ragel is to provide developers with an ability to embed actions into thetransitions and states of a regular expression’s state machine in support of the definition of entire
parsers or large sections of parsers using a single regular expression. From the regular expression
we gain a clear and concise statement of our language. From the state machine we obtain a very
fast and robust executable that lends itself to many kinds of analysis and visualization.

1.3

Overview

Ragel is a language for specifying state machines. The Ragel program is a compiler that assembles
a state machine definition to executable code. Ragel is based on the principle that any regular
language can be converted to a deterministic finite state automaton. Since every regular language
has a state machine representation and vice versa, the terms regular language and state machine(or just machine) will be used interchangeably in this document.
Ragel outputs machines to C, C++, Objective-C, D, Go, Java or Ruby code. The output isdesigned to be generic and is not bound to any particular input or processing method. A Ragelmachine expects to have data passed to it in buffer blocks. When there is no more input, themachine can be queried for acceptance. In this way, a Ragel machine can be used to simply
recognize a regular language like a regular expression library. By embedding code into the regular
language, a Ragel machine can also be used to parse input.
The Ragel language has many operators for constructing and manipulating machines. Machines
are built up from smaller machines, to bigger ones, to the final machine representing the languagethat needs to be recognized or parsed.
The core state machine construction operators are those found in most theory of computation
textbooks. They date back to the 1950s and are widely studied. They are based on set operations
and permit one to think of languages as a set of strings. They are Union, Intersection, Difference,
Concatenation and Kleene Star. Put together, these operators make up what most people know
as regular expressions. Ragel also provides a scanner construction operator and provides operatorsfor explicitly constructing machines using a state chart method. In the state chart method, one
joins machines together without any implied transitions and then explicitly specifies where epsilontransitions should be drawn.
The state machine manipulation operators are specific to Ragel. They allow the programmer

CHAPTER 1. INTRODUCTION

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to access the states and transitions of regular language’s corresponding machine. There are twouses of the manipulation operators. The first and primary use is to embed code into transitions
and states, allowing the programmer to specify the actions of the state machine.
Ragel attempts to make the action embedding facility as intuitive as possible. To do so, anumber of issues need to be addressed. For example, when making a nondeterministic specification
into a DFA using machines that have embedded actions, new transitions are often made that havethe combined actions of several source transitions. Ragel ensures that multiple actions associatedwith a single transition are ordered consistently with respect to the order of reference and thenatural ordering implied by the construction operators.
The second use of the manipulation operators is to assign priorities to transitions. Priorities
provide a convenient way of controlling any nondeterminism introduced by the construction operators. Suppose two transitions leave from the same state and go to distinct target states on the samecharacter. If these transitions are assigned conflicting priorities, then during the determinization
process the transition with the higher priority will take precedence over the transition with thelower priority. The lower priority transition gets abandoned. The transitions would otherwise be
combined into a new transition that goes to a new state that is a combination of the original target
states. Priorities are often required for segmenting machines. The most common uses of priorities
have been encoded into a set of simple operators that should be used instead of priority embeddings
whenever possible.
For the purposes of embedding, Ragel divides transitions and states into different classes. There
are four operators for embedding actions and priorities into the transitions of a state machine. It
is possible to embed into entering transitions, finishing transitions, all transitions and leavingtransitions. The embedding into leaving transitions is a special case. These transition embeddings
get stored in the final states of a machine. They are transferred to any transitions that are madegoing out of the machine by future concatenation or kleene star operations.
There are several more operators for embedding actions into states. Like the transition embeddings, there are various different classes of states that the embedding operators access. For
example, one can access start states, final states or all states, among others. Unlike the transition
embeddings, there are several different types of state action embeddings. These are executed atvarious different times during the processing of input. It is possible to embed actions that areexectued on transitions into a state, on transitions out of a state, on transitions taken on the error
event, or on transitions taken on the EOF event.
Within actions, it is possible to influence the behaviour of the state machine. The user can write
action code that jumps or calls to another portion of the machine, changes the current character
being processed, or breaks out of the processing loop. With the state machine calling feature
Ragel can be used to parse languages that are not regular. For example, one can parse balanced
parentheses by calling into a parser when an open parenthesis character is seen and returning to thestate on the top of the stack when the corresponding closing parenthesis character is seen. Morecomplicated context-free languages such as expressions in C are out of the scope of Ragel.
Ragel also provides a scanner construction operator that can be used to build scanners muchthe same way that Lex is used. The Ragel generated code, which relies on user-defined variables
for backtracking, repeatedly tries to match patterns to the input, favouring longer patterns overshorter ones and patterns that appear ahead of others when the lengths of the possible matches
are identical. When a pattern is matched the associated action is executed.
The key distinguishing feature between scanners in Ragel and scanners in Lex is that Ragelpatterns may be arbitrary Ragel expressions and can therefore contain embedded code. With aRagel-based scanner the user need not wait until the end of a pattern before user code can be
executed.

CHAPTER 1. INTRODUCTION

4Scanners do take Ragel out of the domain of pure state machines and require the user tomaintain the backtracking related variables. However, scanners integrate well with regular statemachine instantiations. They can be called to or jumped to only when needed, or they can becalled out of or jumped out of when a simpler, pure state machine model is appropriate.
Two types of output code style are available. Ragel can produce a table-driven machine or adirectly executable machine. The directly executable machine is much faster than the table-driven.
On the other hand, the table-driven machine is more compact and less demanding on the hostlanguage compiler. It is better suited to compiling large state machines.

1.4

Related WorkLex is perhaps the best-known tool for constructing parsers from regular expressions. In the Lex
processing model, generated code attempts to match one of the user’s regular expression patterns,
favouring longer matches over shorter ones. Once a match is made it then executes the code
associated with the pattern and consumes the matching string. This process is repeated until the
input is fully consumed.
Through the use of start conditions, related sets of patterns may be defined. The active set
may be changed at any time. This allows the user to define different lexical regions. It also allows
the user to link patterns together by requiring that some patterns come before others. This isquite like a concatenation operation. However, use of Lex for languages that require a considerable
amount of pattern concatenation is inappropriate. In such cases a Lex program deteriorates intoa manually specified state machine, where start conditions define the states and pattern actions
define the transitions. Lex is therefore best suited to parsing tasks where the language to be parsed
can be described in terms of regions of tokens.
Lex is useful in many scenarios and has undoubtedly stood the test of time. There are, however, several drawbacks to using Lex. Lex can impose too much overhead for parsing applications
where buffering is not required because all the characters are available in a single string. In thesecases there is structure to the language to be parsed and a parser specification tool can help, but
employing a heavyweight processing loop that imposes a stream “pull” model and dynamic input
buffer allocation is inappropriate. An example of this kind of scenario is the conversion of floating
point numbers contained in a string to their corresponding numerical values.
Another drawback is the very issue that Ragel attempts to solve. It is not possible to executea user action while matching a character contained inside a pattern. For example, if scanning a
programming language and string literals can contain newlines which must be counted, a Lex user
must break up a string literal pattern so as to associate an action with newlines. This forces thedefinition of a new start condition. Alternatively the user can reprocess the text of the matchedstring literal to count newlines.
The Re2C program defines an input processing model similar to that of Lex. Re2C focuseson making generated state machines run very fast and integrate easily into any program, free of
dependencies. Re2C generates directly executable code and is able to claim that generated parsers
run nearly as fast as their hand-coded equivalents. This is very important for user adoption, as
programmers are reluctant to use a tool when a faster alternative exists. A consideration to ease ofuse is also important because developers need the freedom to integrate the generated code as theysee fit.
Many scripting languages provide ways of composing parsers by linking regular expressions
using program logic. For example, Sed and Awk are two established Unix scripting tools thatallow the programmer to exploit regular expressions for the purpose of locating and extracting text

CHAPTER 1. INTRODUCTION

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of interest. High-level programming languages such as Perl, Python, PHP and Ruby all provide
regular expression libraries that allow the user to combine regular expressions with arbitrary code.
In addition to supporting the linking of regular expressions with arbitrary program logic, the Perlprogramming language permits the embedding of code into regular expressions. Perl embeddings do
not translate into the embedding of code into deterministic state machines. Perl regular expressions
are in fact not fully compiled to deterministic machines when embedded code is involved. They are
instead interpreted and involve backtracking. This is shown by the following Perl program. When
it is fed the input abcd the interpretor attempts to match the first alternative, printing a1 b1.
When this possibility fails it backtracks and tries the second possibility, printing a2 b2, at which
point it succeeds.
print "YES\n" if ( <STDIN> =~/( a (?{ print "a1 "; }) b (?{ print "b1 "; }) cX ) |( a (?{ print "a2 "; }) b (?{ print "b2 "; }) cd )/x )In Ragel there is no regular expression interpretor. Aside from the scanner operator, all Ragel
expressions are made into deterministic machines and the run time simply moves from state tostate as it consumes input. An equivalent parser expressed in Ragel would attempt both of thealternatives concurrently, printing a1 a2 b1 b2.

1.5

Development Status

Ragel is a relatively new tool and is under continuous development. As a rough release guide,
minor revision number changes are for implementation improvements and feature additions. Major revision number changes are for implementation and language changes that do not preserve
backwards compatibility. Though in the past this has not always held true: changes that breakcode have crept into minor version number changes. Typically, the documentation lags behind thedevelopment in the interest of documenting only the lasting features. The latest changes are always
documented in the ChangeLog file.

Chapter 2

Constructing State Machines
2.1

Ragel State Machine Specifications

A Ragel input file consists of a program in the host language that contains embedded machine
specifications. Ragel normally passes input straight to output. When it sees a machine specification
it stops to read the Ragel statements and possibly generate code in place of the specification.
Afterwards it continues to pass input through. There can be any number of FSM specifications in
an input file. A multi-line FSM spec starts with %%{ and ends with }%%. A single-line FSM spec
starts with %% and ends at the first newline.
While Ragel is looking for FSM specifications it does basic lexical analysis on the surrounding
input. It interprets literal strings and comments so a %% sequence in either of those will not trigger
the parsing of an FSM specification. Ragel does not pass the input through any preprocessor nor
does it interpret preprocessor directives itself so includes, defines and ifdef logic cannot be used
to alter the parse of a Ragel input file. It is therefore not possible to use an #if 0 directive tocomment out a machine as is commonly done in C code. As an alternative, a machine can beprevented from causing any generated output by commenting out write statements.
In Figure 2.1, a multi-line specification is used to define the machine and single line specifications
are used to trigger the writing of the machine data and execution code.
#include <string.h>#include <stdio.h>

int main( int argc, char **argv ){int cs, res = 0;
if ( argc > 1 ) {char *p = argv[1];char *pe = p + strlen(p) + 1;%% write init;%% write exec;}
printf("result = %i\n", res );return 0;}%%{
machine foo;
main :=( ’foo’ | ’bar’ )0 @{ res = 1; };}%%%% write data;

Figure 2.1: Parsing a command line argument.

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CHAPTER 2. CONSTRUCTING STATE MACHINES

2.1.1

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Naming Ragel Blocks

machine fsm_name;
The machine statement gives the name of the FSM. If present in a specification, this statement
must appear first. If a machine specification does not have a name then Ragel uses the previous
specification name. If no previous specification name exists then this is an error. Because FSM
specifications persist in memory, a machine’s statements can be spread across multiple machine
specifications. This allows one to break up a machine across several files or draw in statementsthat are common to multiple machines using the include statement.

2.1.2

Machine Definition<name> = <expression>;
The machine definition statement associates an FSM expression with a name. Machine expressions assigned to names can later be referenced in other expressions. A definition statement on its
own does not cause any states to be generated. It is simply a description of a machine to be usedlater. States are generated only when a definition is instantiated, which happens when a definition
is referenced in an instantiated expression.

2.1.3

Machine Instantiation<name> := <expression>;
The machine instantiation statement generates a set of states representing an expression. Eachinstantiation generates a distinct set of states. The starting state of the instantiation is written
in the data section of the generated code using the instantiation name. If a machine named mainis instantiated, its start state is used as the specification’s start state and is assigned to the cs
variable by the write init command. If no main machine is given, the start state of the lastmachine instantiation to appear is used as the specification’s start state.
From outside the execution loop, control may be passed to any machine by assigning the entry
point to the cs variable. From inside the execution loop, control may be passed to any machine
instantiation using fcall, fgoto or fnext statements.

2.1.4

Including Ragel Codeinclude FsmName "inputfile.rl";
The include statement can be used to draw in the statements of another FSM specification.
Both the name and input file are optional, however at least one must be given. Without an FSM
name, the given input file is searched for an FSM of the same name as the current specification.
Without an input file the current file is searched for a machine of the given name. If both arepresent, the given input file is searched for a machine of the given name.
Ragel searches for included files from the location of the current file. Additional directories canbe added to the search path using the -I option.

2.1.5

Importing Definitions

import "inputfile.h";CHAPTER 2. CONSTRUCTING STATE MACHINES

8

The import statement scrapes a file for sequences of tokens that match the following forms.
Ragel treats these forms as state machine definitions.
• name ’=’ number• name ’=’ lit_string• ’define’ name number• ’define’ name lit_string
If the input file is a Ragel program then tokens inside any Ragel specifications are ignored. See
Section 5.8.5 for a description of exporting machine definitions.
Ragel searches for imported files from the location of the current file. Additional directories
can be added to the search path using the -I option.

2.2

Lexical Analysis of a Ragel Block

Within a machine specification the following lexical rules apply to the input.
• The # symbol begins a comment that terminates at the next newline.
• The symbols "", ’’, //, [] behave as the delimiters of literal strings. Within them, thefollowing escape sequences are interpreted:\0 \a \b \t \n \v \f \r
A backslash at the end of a line joins the following line onto the current. A backslash preceding
any other character removes special meaning. This applies to terminating characters and to
special characters in regular expression literals. As an exception, regular expression literals
do not support escape sequences as the operands of a range within a list. See the bullet on
regular expressions in Section 2.3.
• The symbols {} delimit a block of host language code that will be embedded into the machineas an action. Within the block of host language code, basic lexical analysis of comments andstrings is done in order to correctly find the closing brace of the block. With the exceptionof FSM commands embedded in code blocks, the entire block is preserved as is for identical
reproduction in the output code.
• The pattern [+-]?[0-9]+ denotes an integer in decimal format. Integers used for specifying
machines may be negative only if the alphabet type is signed. Integers used for specifying
priorities may be positive or negative.
• The pattern 0x[0-9A-Fa-f]+ denotes an integer in hexadecimal format.
• The keywords are access, action, alphtype, getkey, write, machine and include.
• The pattern [a-zA-Z_][a-zA-Z_0-9]* denotes an identifier.
• Any amount of whitespace may separate tokens.

2.3

Basic Machines

The basic machines are the base operands of regular language expressions. They are the smallest
unit to which machine construction and manipulation operators can be applied.

CHAPTER 2. CONSTRUCTING STATE MACHINES

9• ’hello’ – Concatenation Literal. Produces a machine that matches the sequence of characters in the quoted string. If there are 5 characters there will be 6 states chained together
with the characters in the string. See Section 2.2 for information on valid escape sequences.
IN

0

’h’

’e’

1

2

’l’

’l’

3

4

’o’

5

It is possible to make a concatenation literal case-insensitive by appending an i to the string,for example ’cmd’i.
• "hello" – Identical to the single quoted version.
• [hello] – Or Expression. Produces a union of characters. There will be two states with atransition for each unique character between the two states. The [] delimiters behave likethe quotes of a literal string. For example, [ \t] means tab or space. The or expression
supports character ranges with the - symbol as a separator. The meaning of the union canbe negated using an initial ^ character as in standard regular expressions. See Section 2.2 for
information on valid escape sequences in or expressions.
IN’e’, ’h’, ’l’, ’o’

01• ’’, "", and [] – Zero Length Machine. Produces a machine that matches the zero length
string. Zero length machines have one state that is both a start state and a final state.
IN0• 42 – Numerical Literal. Produces a two state machine with one transition on the given
number. The number may be in decimal or hexadecimal format and should be in the range
allowed by the alphabet type. The minimum and maximum values permitted are defined by
the host machine that Ragel is compiled on. For example, numbers in a short alphabet on
an i386 machine should be in the range -32768 to 32767.
IN

0

42

1

• /simple_regex/ – Regular Expression. Regular expressions are parsed as a series of expressions that are concatenated together. Each concatenated expression may be a literalcharacter, the “any” character specified by the . symbol, or a union of characters specified
by the [] delimiters. If the first character of a union is ^ then it matches any character notin the list. Within a union, a range of characters can be given by separating the first andlast characters of the range with the - symbol. Each concatenated machine may have repetition specified by following it with the * symbol. The standard escape sequences describedin Section 2.2 are supported everywhere in regular expressions except as the operands of arange within in a list. This notation also supports the i trailing option. Use it to produce
case-insensitive machines, as in /GET/i.

CHAPTER 2. CONSTRUCTING STATE MACHINES

10

Ragel does not support very complex regular expressions because the desired results canalways be achieved using the more general machine construction operators listed in Section
2.5. The following diagram shows the result of compiling /ab*[c-z].*[123]/. DEF represents
the default transition, which is taken if no other transition can be taken.
’b’

IN

0

’a’

’1’..’3’

DEF’c’..’z’

1’1’..’3’
2

3
DEF• ’a’ .. ’z’ – Range. Produces a machine that matches any characters in the specified range.
Allowable upper and lower bounds of the range are concatenation literals of length one andnumerical literals. For example, 0x10..0x20, 0..63, and ’a’..’z’ are valid ranges. The
bounds should be in the range allowed by the alphabet type.
IN0’a’..’z’1• variable_name – Lookup the machine definition assigned to the variable name given and use
an instance of it. See Section 2.1.2 for an important note on what it means to reference avariable name.
• builtin_machine – There are several built-in machines available for use. They are all two
state machines for the purpose of matching common classes of characters. They are:– any

– Any character in the alphabet.

– ascii – Ascii characters. 0..127– extend – Ascii extended characters. This is the range -128..127 for signed alphabets
and the range 0..255 for unsigned alphabets.
– alpha – Alphabetic characters. [A-Za-z]– digit – Digits. [0-9]– alnum – Alpha numerics. [0-9A-Za-z]– lower – Lowercase characters. [a-z]– upper – Uppercase characters. [A-Z]– xdigit – Hexadecimal digits. [0-9A-Fa-f]– cntrl – Control characters. 0..31– graph – Graphical characters. [!-~]– print – Printable characters. [ -~]– punct – Punctuation. Graphical characters that are not alphanumerics. [!-/:-@[-‘{-~]– space – Whitespace. [\t\v\f\n\r ]– zlen– Zero length string. ""– empty – Empty set. Matches nothing. ^any

CHAPTER 2. CONSTRUCTING STATE MACHINES

2.4

11

Operator Precedence

The following table shows operator precedence from lowest to highest. Operators in the same
precedence group are evaluated from left to right.
1
2
3
4
5

6

7
8
9

2.5

,
| & - -. <: :> :>>:->> @ $ %>/ $/ %/ </ @/ <>/>! $! %! <! @! <>!
>^ $^ %^ <^ @^ <>^>~ $~ %~ <~ @~ <>~>* $* %* <* @* <>** ** ? + {n} {,n} {n,} {n,m}! ^( <expr> )Join
Union, Intersection and Subtraction
Concatenation
Label
Epsilon Transition
Transitions Actions and Priorities
EOF Actions
Global Error Actions
Local Error Actions
To-State Actions
From-State Action
Repetition
Negation and Character-Level Negation
Grouping

Regular Language Operators

When using Ragel it is helpful to have a sense of how it constructs machines. The determinization
process can produce results that seem unusual to someone not familiar with the NFA to DFA
conversion algorithm. In this section we describe Ragel’s state machine operators. Though theoperators are defined using epsilon transitions, it should be noted that this is for discussion only.
The epsilon transitions described in this section do not persist, but are immediately removed by
the determinization process which is executed at every operation. Ragel does not make use of any
nondeterministic intermediate state machines.
To create an epsilon transition between two states x and y is to copy all of the properties ofy into x. This involves drawing in all of y’s to-state actions, EOF actions, etc., in addition to its
transitions. If x and y both have a transition out on the same character, then the transitions must
be combined. During transition combination a new transition is made that goes to a new statethat is the combination of both target states. The new combination state is created using the same
epsilon transition method. The new state has an epsilon transition drawn to all the states thatcompose it. Since the creation of new epsilon transitions may be triggered every time an epsilontransition is drawn, the process of drawing epsilon transitions is repeated until there are no more
epsilon transitions to be made.
A very common error that is made when using Ragel is to make machines that do too much.
That is, to create machines that have unintentional nondetermistic properties. This usually resultsfrom being unaware of the common strings between machines that are combined together using
the regular language operators. This can involve never leaving a machine, causing its actions to be
propagated through all the following states. Or it can involve an alternation where both branches
are unintentionally taken simultaneously.
This problem forces one to think hard about the language that needs to be matched. To guard
against this kind of problem one must ensure that the machine specification is divided up using
boundaries that do not allow ambiguities from one portion of the machine to the next. See Chapter
4 for more on this problem and how to solve it.

CHAPTER 2. CONSTRUCTING STATE MACHINES

12The Graphviz tool is an immense help when debugging improperly compiled machines or otherwise learning how to use Ragel. Graphviz Dot files can be generated from Ragel programs using
the -V option. See Section 2.7 for more information.

2.5.1

Union

expr | expr
The union operation produces a machine that matches any string in machine one or machine
two. The operation first creates a new start state. Epsilon transitions are drawn from the newstart state to the start states of both input machines. The resulting machine has a final state set
equivalent to the union of the final state sets of both input machines. In this operation, there is theopportunity for nondeterminism among both branches. If there are strings, or prefixes of strings
that are matched by both machines then the new machine will follow both parts of the alternation
at once. The union operation is shown below.

εεThe following example demonstrates the union of three machines representing common tokens.
# Hex digits, decimal digits, or identifiers
main := ’0x’ xdigit+ | digit+ | alpha alnum*;’0’..’9’, ’A’..’F’, ’a’..’f’

’x’
1

’0’

IN

’0’..’9’’1’..’9’03’0’..’9’, ’A’..’F’, ’a’..’f’

4’0’..’9’2’A’..’Z’, ’a’..’z’’0’..’9’, ’A’..’Z’, ’a’..’z’

5

2.5.2

Intersection

expr & expr
Intersection produces a machine that matches any string that is in both machine one andCHAPTER 2. CONSTRUCTING STATE MACHINES

13

machine two. To achieve intersection, a union is performed on the two machines. After the result
has been made deterministic, any final state that is not a combination of final states from bothmachines has its final state status revoked. To complete the operation, paths that do not lead toa final state are pruned from the machine. Therefore, if there are any such paths in either of theexpressions they will be removed by the intersection operator. Intersection can be used to requirethat two independent patterns be simultaneously satisfied as in the following example.
# Match lines four characters wide that contain# words separated by whitespace.
main :=/[^\n][^\n][^\n][^\n]\n/* &(/[a-z][a-z]*/ | [ \n])**;’ ’, ’a’..’z’IN

1’ ’, ’a’..’z’

2’ ’, ’a’..’z’

3’ ’, ’a’..’z’

0

4
10

2.5.3

Difference

expr - expr
The difference operation produces a machine that matches strings that are in machine one
but are not in machine two. To achieve subtraction, a union is performed on the two machines.
After the result has been made deterministic, any final state that came from machine two or is acombination of states involving a final state from machine two has its final state status revoked.
As with intersection, the operation is completed by pruning any path that does not lead to a final
state. The following example demonstrates the use of subtraction to exclude specific cases from aset.
# Subtract keywords from identifiers.
main := /[a-z][a-z]*/ - ( ’for’ | ’int’ );’a’..’q’, ’s’..’z’
3

’r’

’o’

4
’a’..’z’

2’a’..’n’, ’p’..’z’
’t’

’f’

’a’..’s’, ’u’..’z’
IN

0

’i’

’n’

6
’a’..’m’, ’o’..’z’

5’a’..’e’, ’g’..’h’, ’j’..’z’

2.5.4

Strong Difference

expr -- expr

’a’..’z’

1CHAPTER 2. CONSTRUCTING STATE MACHINES

14

Strong difference produces a machine that matches any string of the first machine that does nothave any string of the second machine as a substring. In the following example, strong subtraction
is used to excluded CRLF from a sequence. In the corresponding visualization, the label DEF is shortfor default. The default transition is taken if no other transition can be taken.
crlf = ’\r\n’;
main := [a-z]+ ’:’ ( any* -- crlf ) crlf;
DEF’a’..’z’IN

0’a’..’z’

1

13
13

’:’

3

2

10

4

DEF

This operator is equivalent to the following.
expr - ( any* expr any* )2.5.5

Concatenation

expr . expr
Concatenation produces a machine that matches all the strings in machine one followed by all
the strings in machine two. Concatenation draws epsilon transitions from the final states of thefirst machine to the start state of the second machine. The final states of the first machine lose
their final state status, unless the start state of the second machine is final as well. Concatenationis the default operator. Two machines next to each other with no operator between them resultsin concatenation.
εεThe opportunity for nondeterministic behaviour results from the possibility of the final states ofthe first machine accepting a string that is also accepted by the start state of the second machine.
The most common scenario in which this happens is the concatenation of a machine that repeatssome pattern with a machine that gives a terminating string, but the repetition machine does not
exclude the terminating string. The example in Section 2.5.4 guards against this. Another example
is the expression ("’" any* "’"). When executed the thread of control will never leave the any*machine. This is a problem especially if actions are embedded to process the characters of the any*
component.
In the following example, the first machine is always active due to the nondeterministic natureof concatenation. This particular nondeterminism is intended however because we wish to permit
EOF strings before the end of the input.
# Require an eof marker on the last line.
main := /[^\n]*\n/* . ’EOF\n’;CHAPTER 2. CONSTRUCTING STATE MACHINES

15

DEF

10

DEF

3

DEF

’F’

’O’
DEF

4
10

10

10

2

DEF

’E’

1

’E’
IN

5
0

10

DEF
10

Note: There is a language ambiguity involving concatenation and subtraction. Because concatenation is the default operator for two adjacent machines there is an ambiguity between subtraction ofa positive numerical literal and concatenation of a negative numerical literal. For example, (x-7)
could be interpreted as (x . -7) or (x - 7). In the Ragel language, the subtraction operator
always takes precedence over concatenation of a negative literal. We adhere to the rule that thedefault concatenation operator takes effect only when there are no other operators between two
machines. Beware of writing machines such as (any -1) when what is desired is a concatenation of
any and -1. Instead write (any . -1) or (any (-1)). If in doubt of the meaning of your program
do not rely on the default concatenation operator; always use the . symbol.

2.5.6

Kleene Star

expr*
The machine resulting from the Kleene Star operator will match zero or more repetitions of
the machine it is applied to. It creates a new start state and an additional final state. Epsilontransitions are drawn between the new start state and the old start state, between the new start
state and the new final state, and between the final states of the machine and the new start state.
After the machine is made deterministic the effect is of the final states getting all the transitionsof the start state.

ε
ε

ε
ε

The possibility for nondeterministic behaviour arises if the final states have transitions on any of
the same characters as the start state. This is common when applying kleene star to an alternationof tokens. Like the other problems arising from nondeterministic behavior, this is discussed in
more detail in Chapter 4. This particular problem can also be solved by using the longest-match
construction discussed in Section 6.3 on scanners.
In this example, there is no nondeterminism introduced by the exterior kleene star due to theCHAPTER 2. CONSTRUCTING STATE MACHINES

16

newline at the end of the regular expression. Without the newline the exterior kleene star would be
redundant and there would be ambiguity between repeating the inner range of the regular expression
and the entire regular expression. Though it would not cause a problem in this case, unnecessary
nondeterminism in the kleene star operator often causes undesired results for new Ragel users andmust be guarded against.
# Match any number of lines with only lowercase letters.
main := /[a-z]*\n/*;10IN’a’..’z’’a’..’z’
1

0
10

2.5.7

One Or More Repetition

expr+
This operator produces the concatenation of the machine with the kleene star of itself. The
result will match one or more repetitions of the machine. The plus operator is equivalent to(expr . expr*).
# Match alpha-numeric words.
main := alnum+;’0’..’9’, ’A’..’Z’, ’a’..’z’

IN

2.5.8

0

’0’..’9’, ’A’..’Z’, ’a’..’z’

1

Optional

expr?
The optional operator produces a machine that accepts the machine given or the zero length
string. The optional operator is equivalent to (expr | ’’ ). In the following example the optional
operator is used to possibly extend a token.
# Match integers or floats.
main := digit+ (’.’ digit+)?;’0’..’9’IN0’0’..’9’

1’0’..’9’’.’2’0’..’9’3

CHAPTER 2. CONSTRUCTING STATE MACHINES

2.5.9

Repetition

expr {n}expr {,n}expr {n,}expr {n,m}2.5.1017– Exactly N copies of expr.
– Zero to N copies of expr.
– N or more copies of expr.
– N to M copies of expr.

Negation!expr
Negation produces a machine that matches any string not matched by the given machine.
Negation is equivalent to (any* - expr).
# Accept anything but a string beginning with a digit.
main := ! ( digit any* );DEFIN

2.5.110−128..’/’, ’:’..127

1

Character-Level Negation^expr
Character-level negation produces a machine that matches any single character not matched by
the given machine. Character-Level Negation is equivalent to (any - expr). It must be applied
only to machines that match strings of length one.

2.6

State Machine Minimization

State machine minimization is the process of finding the minimal equivalent FSM accepting thelanguage. Minimization reduces the number of states in machines by merging equivalent states. It
does not change the behaviour of the machine in any way. It will cause some states to be merged
into one because they are functionally equivalent. State minimization is on by default. It can beturned off with the -n option.
The algorithm implemented is similar to Hopcroft’s state minimization algorithm. Hopcroft’s
algorithm assumes a finite alphabet that can be listed in memory, whereas Ragel supports arbitrary
integer alphabets that cannot be listed in memory. Though exact analysis is very difficult, Ragel
minimization runs close to O(n × log(n)) and requires O(n) temporary storage where n is thenumber of states.

2.7

Visualization

Ragel is able to emit compiled state machines in Graphviz’s Dot file format. This is done using
the -V option. Graphviz support allows users to perform incremental visualization of their parsers.
User actions are displayed on transition labels of the graph.

CHAPTER 2. CONSTRUCTING STATE MACHINES

18

If the final graph is too large to be meaningful, or even drawn, the user is able to inspectportions of the parser by naming particular regular expression definitions with the -S and -M options
to the ragel program. Use of Graphviz greatly improves the Ragel programming experience. It
allows users to learn Ragel by experimentation and also to track down bugs caused by unintended
nondeterminism.
Ragel has another option to help debugging. The -x option causes Ragel to emit the compiled
machine in an XML format.

Chapter 3

User Actions
Ragel permits the user to embed actions into the transitions of a regular expression’s corresponding
state machine. These actions are executed when the generated code moves over a transition. Like
the regular expression operators, the action embedding operators are fully compositional. They
take a state machine and an action as input, embed the action and yield a new state machine thatcan be used in the construction of other machines. Due to the compositional nature of embeddings,
the user has complete freedom in the placement of actions.
A machine’s transitions are categorized into four classes. The action embedding operators accessthe transitions defined by these classes. The entering transition operator > isolates the start state,then embeds an action into all transitions leaving it. The finishing transition operator @ embeds
an action into all transitions going into a final state. The all transition operator $ embeds anaction into all transitions of an expression. The leaving transition operator % provides access to the
yet-unmade transitions moving out of the machine via the final states.

3.1

Embedding Actions

action ActionName {/* Code an action here. */count += 1;}
The action statement defines a block of code that can be embedded into an FSM. Action names
can be referenced by the action embedding operators in expressions. Though actions need not
be named in this way (literal blocks of code can be embedded directly when building machines),
defining reusable blocks of code whenever possible is good practice because it potentially increases
the degree to which the machine can be minimized.
Within an action some Ragel expressions and statements are parsed and translated. These
allow the user to interact with the machine from action code. See Section 3.4 for a complete list of
statements and values available in code blocks.

3.1.1

Entering Action

expr > action
The entering action operator embeds an action into all transitions that enter into the machinefrom the start state. If the start state is final, then the action is also embedded into the start state19

CHAPTER 3. USER ACTIONS

20

as a leaving action. This means that if a machine accepts the zero-length string and control passes
through the start state then the entering action is executed. Note that this can happen on both afollowing character and on the EOF event.
In some machines the start state has transtions coming in from within the machine. In thesecases the start state is first isolated from the rest of the machine ensuring that the entering actions
are exected once only.
# Execute A at the beginning of a string of alpha.
action A {}main := ( lower* >A ) . ’ ’;
SP / AIN
1

3’a’..’z’ / A’a’..’z’

SP

2

3.1.2

Finishing Action

expr @ action
The finishing action operator embeds an action into any transitions that move the machine
into a final state. Further input may move the machine out of the final state, but keep it in themachine. Therefore finishing actions may be executed more than once if a machine has any internaltransitions out of a final state. In the following example the final state has no transitions out andthe finishing action is executed only once.
# Execute A when the trailing space is seen.
main := ( lower* ’ ’ ) @A;’a’..’z’

IN

3.1.3

0

’’/A

1

All Transition Action

expr $ action
The all transition operator embeds an action into all transitions of a machine. The action is
executed whenever a transition of the machine is taken. In the following example, A is executedon every character matched.
# Execute A on any characters of the machine.
main := ( ’m1’ | ’m2’ ) $A;

CHAPTER 3. USER ACTIONS
IN

3.1.4

21
’m’ / A0’1’..’2’ / A

1

2

Leaving Actions

expr % action
The leaving action operator queues an action for embedding into the transitions that go outof a machine via a final state. The action is first stored in the machine’s final states and islater transferred to any transitions that are made going out of the machine by a kleene star or
concatenation operation.
If a final state of the machine is still final when compilation is complete then the leaving actionis also embedded as an EOF action. Therefore, leaving the machine is defined as either leaving ona character or as state machine acceptance.
This operator allows one to associate an action with the termination of a sequence, without
being concerned about what particular character terminates the sequence. In the following example,
A is executed when leaving the alpha machine on the newline character.
# Match a word followed by a newline. Execute A when
# finishing the word.
main := ( lower+ %A ) . ’\n’;’a’..’z’IN

0’a’..’z’

1

10 / A

2

In the following example, the term_word action could be used to register the appearance of aword and to clear the buffer that the lower action used to store the text of it.
word = ( [a-z] @lower )+ %term_word;
main := word ( ’ ’ @space word )* ’\n’ @newline;’a’..’z’ / lowerIN0’a’..’z’ / lower’ ’ / term_word, space

1

10 / term_word, newline

2

In this final example of the action embedding operators, A is executed upon entering the alphamachine, B is executed on all transitions of the alpha machine, C is executed when the alphamachine is exited by moving into the newline machine and N is executed when the newline machine
moves into a final state.
# Execute A on starting the alpha machine, B on every transition# moving through it and C upon finishing. Execute N on the newline.
main := ( lower* >A $B %C ) . ’\n’ @N;

CHAPTER 3. USER ACTIONS

22’\n’ / A, C, NIN
1’a’..’z’ / A, B

3’a’..’z’ / B’\n’ / C, N

2

3.2

State Action Embedding Operators

The state embedding operators allow one to embed actions into states. Like the transition embedding operators, there are several different classes of states that the operators access. The meaningsof the symbols are similar to the meanings of the symbols used for the transition embedding operators. The design of the state selections was driven by a need to cover the states of an expression
with exactly one error action.
Unlike the transition embedding operators, the state embedding operators are also distinguishedby the different kinds of events that embedded actions can be associated with. Therefore the stateembedding operators have two components. The first, which is the first one or two characters,
specifies the class of states that the action will be embedded into. The second component specifies
the type of event the action will be executed on. The symbols of the second component also haveequivalent kewords.
The different classes of states are:• > – the start state• < – any state except the start state• $ – all states• % – final states• @ – any state except final states• <> – any except start and final (middle)

3.2.1

The different kinds of embeddings are:• ~ – to-state actions (to)• * – from-state actions (from)• / – EOF actions (eof)• ! – error actions (err)• ^ – local error actions (lerr)

To-State and From-State Actions

To-State Actions>~action<~action$~action%~action@~action<>~action>to(name)<to(name)$to(name)%to(name)@to(name)<>to(name)>to{...} – the start state<to{...} – any state except the start state$to{...} – all states%to{...} – final states@to{...} – any state except final states<>to{...} – any except start and final (middle)

To-state actions are executed whenever the state machine moves into the specified state, either
by a natural movement over a transition or by an action-based transfer of control such as fgoto.
They are executed after the in-transition’s actions but before the current character is advanced
and tested against the end of the input block. To-state embeddings stay with the state. They areirrespective of the state’s current set of transitions and any future transitions that may be addedin or out of the state.
Note that the setting of the current state variable cs outside of the execute code is not consideredby Ragel as moving into a state and consequently the to-state actions of the new current state are

CHAPTER 3. USER ACTIONS

23

not executed. This includes the initialization of the current state when the machine begins. This isbecause the entry point into the machine execution code is after the execution of to-state actions.
From-State Actions>*action<*action$*action%*action@*action<>*action>from(name)<from(name)$from(name)%from(name)@from(name)<>from(name)>from{...} – the start state<from{...} – any state except the start state$from{...} – all states%from{...} – final states@from{...} – any state except final states<>from{...} – any except start and final (middle)

From-state actions are executed whenever the state machine takes a transition from a state,either to itself or to some other state. These actions are executed immediately after the current
character is tested against the input block end marker and before the transition to take is sought
based on the current character. From-state actions are therefore executed even if a transition
cannot be found and the machine moves into the error state. Like to-state embeddings, from-stateembeddings stay with the state.

3.2.2

EOF Actions>/action</action$/action%/action@/action<>/action>eof(name)<eof(name)$eof(name)%eof(name)@eof(name)<>eof(name)>eof{...} – the start state<eof{...} – any state except the start state$eof{...} – all states%eof{...} – final states@eof{...} – any state except final states<>eof{...} – any except start and final (middle)

The EOF action embedding operators enable the user to embed actions that are executed at
the end of the input stream. EOF actions are stored in states and generated in the write exec
block. They are run when p == pe == eof as the execute block is finishing. EOF actions are free
to adjust p and jump to another part of the machine to restart execution.

3.2.3

Handling Errors

In many applications it is useful to be able to react to parsing errors. The user may wish to print anerror message that depends on the context. It may also be desirable to consume input in an attempt
to return the input stream to some known state and resume parsing. To support error handling andrecovery, Ragel provides error action embedding operators. There are two kinds of error actions:
global error actions and local error actions. Error actions can be used to simply report errors, orby jumping to a machine instantiation that consumes input, can attempt to recover from errors.
Global Error Actions>!action<!action$!action%!action>err(name)<err(name)$err(name)%err(name)>err{...}<err{...}$err{...}%err{...}– the start state– any state except the start state– all states– final states

CHAPTER 3. USER ACTIONS@!action<>!action@err(name)<>err(name)24@err{...} – any state except final states<>err{...} – any except start and final (middle)

Global error actions are stored in the states they are embedded into until compilation is complete. They are then transferred to the transitions that move into the error state. These transitions
are taken on all input characters that are not already covered by the state’s transitions. If a statewith an error action is not final when compilation is complete, then the action is also embedded as
an EOF action.
Error actions can be used to recover from errors by jumping back into the machine with fgoto
and optionally altering p.
Local Error Actions>^action<^action$^action%^action@^action<>^action>lerr(name)<lerr(name)$lerr(name)%lerr(name)@lerr(name)<>lerr(name)>lerr{...} – the start state<lerr{...} – any state except the start state$lerr{...} – all states%lerr{...} – final states@lerr{...} – any state except final states<>lerr{...} – any except start and final (middle)

Like global error actions, local error actions are also stored in the states they are embedded into
until a transfer point. The transfer point is different however. Each local error action embedding
is associated with a name. When a machine definition has been fully constructed, all local error
action embeddings associated with the same name as the machine definition are transferred to theerror transitions. At this time they are also embedded as EOF actions in the case of non-final
states.
Local error actions can be used to specify an action to take when a particular section of a larger
state machine fails to match. A particular machine definition’s “thread” may die and the local
error actions executed, however the machine as a whole may continue to match input.
There are two forms of local error action embeddings. In the first form the name defaults tothe current machine. In the second form the machine name can be specified. This is useful when
it is more convenient to specify the local error action in a sub-definition that is used to construct
the machine definition that the local error action is associated with. To embed local error actions
and explicitly state the machine definition on which the transfer is to happen use (name, action)
as the action.
ExampleThe following example uses error actions to report an error and jump to a machine that consumes
the remainder of the line when parsing fails. After consuming the line, the error recovery machine
returns to the main loop.
action cmd_err {printf( "command error\n" );fhold; fgoto line;}
action from_err {printf( "from error\n" );fhold; fgoto line;}

CHAPTER 3. USER ACTIONS

25

action to_err {printf( "to error\n" );fhold; fgoto line;}
line := [^\n]* ’\n’ @{ fgoto main; };main := ((’from’ @err(cmd_err)( ws+ address ws+ date ’\n’ ) $err(from_err) |’to’ @err(cmd_err)( ws+ address ’\n’ ) $err(to_err)
)
)*;

3.3

Action Ordering and Duplicates

When combining expressions that have embedded actions it is often the case that a number ofactions must be executed on a single input character. For example, following a concatenation theleaving action of the left expression and the entering action of the right expression will be embedded
into one transition. This requires a method of ordering actions that is intuitive and predictable forthe user, and repeatable for the compiler.
We associate with the embedding of each action a unique timestamp that is used to orderactions that appear together on a single transition in the final state machine. To accomplish
this we recursively traverse the parse tree of regular expressions and assign timestamps to action
embeddings. References to machine definitions are followed in the traversal. When we visit a parsetree node we assign timestamps to all entering action embeddings, recurse on the parse tree, thenassign timestamps to the remaining all, finishing, and leaving embeddings in the order in which
they appear.
By default Ragel does not permit a single action to appear multiple times in an action list. Whenthe final machine has been created, actions that appear more than once in a single transition, tostate, from-state or EOF action list have their duplicates removed. The first appearance of theaction is preserved. This is useful in a number of scenarios. First, it allows us to union machineswith common prefixes without worrying about the action embeddings in the prefix being duplicated.
Second, it prevents leaving actions from being transferred multiple times. This can happen whena machine is repeated, then followed with another machine that begins with a common character.
For example:
word = [a-z]+ %act;
main := word ( ’\n’ word )* ’\n\n’;Note that Ragel does not compare action bodies to determine if they have identical program
text. It simply checks for duplicates using each action block’s unique location in the program.
The removal of duplicates can be turned off using the -d option.

3.4

Values and Statements Available in Code Blocks

The following values are available in code blocks:

CHAPTER 3. USER ACTIONS

26• fpc – A pointer to the current character. This is equivalent to accessing the p variable.
• fc – The current character. This is equivalent to the expression (*p).
• fcurs – An integer value representing the current state. This value should only be read from.
To move to a different place in the machine from action code use the fgoto, fnext or fcall
statements. Outside of the machine execution code the cs variable may be modified.
• ftargs – An integer value representing the target state. This value should only be read from.
Again, fgoto, fnext and fcall can be used to move to a specific entry point.
• fentry(<label>) – Retrieve an integer value representing the entry point label. The integer
value returned will be a compile time constant. This number is suitable for later use in control
flow transfer statements that take an expression. This value should not be compared against
the current state because any given label can have multiple states representing it. The value
returned by fentry can be any one of the multiple states that it represents.
The following statements are available in code blocks:• fhold; – Do not advance over the current character. If processing data in multiple buffer
blocks, the fhold statement should only be used once in the set of actions executed on acharacter. Multiple calls may result in backing up over the beginning of the buffer block.
The fhold statement does not imply any transfer of control. It is equivalent to the p--;statement.
• fexec <expr>; – Set the next character to process. This can be used to backtrack to previous
input or advance ahead. Unlike fhold, which can be used anywhere, fexec requires the user
to ensure that the target of the backtrack is in the current buffer block or is known tobe somewhere ahead of it. The machine will continue iterating forward until pe is arrived
at, fbreak is called or the machine moves into the error state. In actions embedded intotransitions, the fexec statement is equivalent to setting p to one position ahead of the next
character to process. If the user also modifies pe, it is possible to change the buffer block
entirely.
• fgoto <label>; – Jump to an entry point defined by <label>. The fgoto statement immediately transfers control to the destination state.
• fgoto *<expr>; – Jump to an entry point given by <expr>. The expression must evaluate
to an integer value representing a state.
• fnext <label>; – Set the next state to be the entry point defined by label. The fnext
statement does not immediately jump to the specified state. Any action code following thestatement is executed.
• fnext *<expr>; – Set the next state to be the entry point given by <expr>. The expression
must evaluate to an integer value representing a state.
• fcall <label>; – Push the target state and jump to the entry point defined by <label>.
The next fret will jump to the target of the transition on which the call was made. Useof fcall requires the declaration of a call stack. An array of integers named stack and asingle integer named top must be declared. With the fcall construct, control is immediatelytransferred to the destination state. See section 6.1 for more information.

CHAPTER 3. USER ACTIONS

27• fcall *<expr>; – Push the current state and jump to the entry point given by <expr>. The
expression must evaluate to an integer value representing a state.
• fret; – Return to the target state of the transition on which the last fcall was made. Useof fret requires the declaration of a call stack. Control is immediately transferred to thedestination state.
• fbreak; – Advance p, save the target state to cs and immediately break out of the executeloop. This statement is useful in conjunction with the noend write option. Rather thanprocess input until pe is arrived at, the fbreak statement can be used to stop processing froman action. After an fbreak statement the p variable will point to the next character in the
input. The current state will be the target of the current transition. Note that fbreak causes
the target state’s to-state actions to be skipped.
Note: Once actions with control-flow commands are embedded into a machine, the user must
exercise caution when using the machine as the operand to other machine construction operators.
If an action jumps to another state then unioning any transition that executes that action withanother transition that follows some other path will cause that other path to be lost. Using
commands that manually jump around a machine takes us out of the domain of regular languages
because transitions that the machine construction operators are not aware of are introduced. These
commands should therefore be used with caution.

Chapter 4

Controlling Nondeterminism
Along with the flexibility of arbitrary action embeddings comes a need to control nondeterminism
in regular expressions. If a regular expression is ambiguous, then sub-components of a parser otherthan the intended parts may become active. This means that actions that are irrelevant to thecurrent subset of the parser may be executed, causing problems for the programmer.
Tools that are based on regular expression engines and that are used for recognition tasks willusually function as intended regardless of the presence of ambiguities. It is quite common for usersof scripting languages to write regular expressions that are heavily ambiguous and it generally does
not matter. As long as one of the potential matches is recognized, there can be any number of othermatches present. In some parsing systems the run-time engine can employ a strategy for resolving
ambiguities, for example always pursuing the longest possible match and discarding others.
In Ragel, there is no regular expression run-time engine, just a simple state machine execution
model. When we begin to embed actions and face the possibility of spurious action execution, it
becomes clear that controlling nondeterminism at the machine construction level is very important.
Consider the following example.
ws = [\n\t ];line = word $first ( ws word $tail )* ’\n’;lines = line*;’a’..’z’ / tail10’a’..’z’ / first, tail’a’..’z’ / first

4
10

IN

0

’a’..’z’ / first’a’..’z’ / tail3’a’..’z’ / tail, first
10

1

5

9, ’ ’

2

9, ’ ’

9, ’ ’

Since the ws expression includes the newline character, we will not finish the line expression
when a newline character is seen. We will simultaneously pursue the possibility of matching further
words on the same line and the possibility of matching a second line. Evidence of this fact is in thestate tables. On several transitions both the first and tail actions are executed. The solution
here is simple: exclude the newline character from the ws expression.
ws = [\t ];line = word $first ( ws word $tail )* ’\n’;lines = line*;28

CHAPTER 4. CONTROLLING NONDETERMINISM

29’a’..’z’ / first’a’..’z’ / tail’a’..’z’ / firstIN

9, ’ ’

1’a’..’z’ / tail2

9, ’ ’

10

0

3

10

Solving this kind of problem is straightforward when the ambiguity is created by strings thatare a single character long. When the ambiguity is created by strings that are multiple characterslong we have a more difficult problem. The following example is an incorrect attempt at a regularexpression for C language comments.
comment = ’/*’ ( any @comm )* ’*/’;
main := comment ’ ’;
DEF / comm
DEF / comm

IN

0

’/’

’*’ / comm5’ ’ / comm’*’ / comm’*’1’*’ / comm2

3

DEF / comm’/’ / comm’*’ / comm4

DEF / commUsing standard concatenation, we will never leave the any* expression. We will forever entertain the possibility that a ’*/’ string that we see is contained in a longer comment and that,simultaneously, the comment has ended. The concatenation of the comment machine with SP isdone to show this. When we match space, we are also still matching the comment body.
One way to approach the problem is to exclude the terminating string from the any* expression
using set difference. We must be careful to exclude not just the terminating string, but any stringthat contains it as a substring. A verbose, but proper specification of a C comment parser is givenby the following regular expression.
comment = ’/*’ ( ( any @comm )* - ( any* ’*/’ any* ) ) ’*/’;
DEF / comm

IN

0

’/’

1

’*’

’*’ / comm’*’ / comm
3

2

’/’

4

DEF / comm

Note that Ragel’s strong subtraction operator -- can also be used here. In doing this subtraction
we have phrased the problem of controlling non-determinism in terms of excluding strings common
to two expressions that interact when combined. We can also phrase the problem in terms ofthe transitions of the state machines that implement these expressions. During the concatenationof any* and ’*/’ we will be making transitions that are composed of both the loop of the firstexpression and the final character of the second. At this time we want the transition on the ’/’character to take precedence over and disallow the transition that originated in the any* loop.
In another parsing problem, we wish to implement a lightweight tokenizer that we can utilize
in the composition of a larger machine. For example, some HTTP headers have a token stream asa sub-language. The following example is an attempt at a regular expression-based tokenizer that
does not function correctly due to unintended nondeterminism.

CHAPTER 4. CONTROLLING NONDETERMINISM

30

header_contents = (lower+ >start_str $on_char %finish_str |
’ ’
)*;
’’

IN

0

’a’..’z’ / on_char, finish_str, start_str’a’..’z’ / start_str, on_char

1’ ’ / finish_str

In this case, the problem with using a standard kleene star operation is that there is an ambiguity
between extending a token and wrapping around the machine to begin a new token. Using thestandard operator, we get an undesirable nondeterministic behaviour. Evidence of this can be seenon the transition out of state one to itself. The transition extends the string, and simultaneously,
finishes the string only to immediately begin a new one. What is required is for the transitions
that represent an extension of a token to take precedence over the transitions that represent the
beginning of a new token. For this problem there is no simple solution that uses standard regularexpression operators.

4.1

Priorities

A priority mechanism was devised and built into the determinization process, specifically for the
purpose of allowing the user to control nondeterminism. Priorities are integer values embedded
into transitions. When the determinization process is combining transitions that have different
priorities, the transition with the higher priority is preserved and the transition with the lower
priority is dropped.
Unfortunately, priorities can have unintended side effects because their operation requires thatthey linger in transitions indefinitely. They must linger because the Ragel program cannot know
when the user is finished with a priority embedding. A solution whereby they are explicitly deleted
after use is conceivable; however this is not very user-friendly. Priorities were therefore madeinto named entities. Only priorities with the same name are allowed to interact. This allows any
number of priorities to coexist in one machine for the purpose of controlling various different regularexpression operations and eliminates the need to ever delete them. Such a scheme allows the user
to choose a unique name, embed two different priority values using that name and be confident
that the priority embedding will be free of any side effects.
In the first form of priority embedding the name defaults to the name of the machine definition
that the priority is assigned in. In this sense priorities are by default local to the current machine
definition or instantiation. Beware of using this form in a longest-match machine, since there is
only one name for the entire set of longest match patterns. In the second form the priority’s namecan be specified, allowing priority interaction across machine definition boundaries.
• expr > int – Sets starting transitions to have priority int.
• expr @ int – Sets transitions that go into a final state to have priority int.
• expr $ int – Sets all transitions to have priority int.
• expr % int – Sets leaving transitions to have priority int. When a transition is made goingout of the machine (either by concatenation or kleene star) its priority is immediately set tothe leaving priority.

CHAPTER 4. CONTROLLING NONDETERMINISM

31

The second form of priority assignment allows the programmer to specify the name to which
the priority is assigned.
• expr > (name, int) – Starting transitions.
• expr @ (name, int) – Finishing transitions (into a final state).
• expr $ (name, int) – All transitions.
• expr % (name, int) – Leaving transitions.

4.2

Guarded Operators that Encapsulate Priorities

Priority embeddings are a very expressive mechanism. At the same time they can be very confusing
for the user. They force the user to imagine the transitions inside two interacting expressions andwork out the precise effects of the operations between them. When we consider that this problem
is worsened by the potential for side effects caused by unintended priority name collisions, we seethat exposing the user to priorities is undesirable.
Fortunately, in practice the use of priorities has been necessary only in a small number ofscenarios. This allows us to encapsulate their functionality into a small set of operators and fully
hide them from the user. This is advantageous from a language design point of view because it
greatly simplifies the design.
Going back to the C comment example, we can now properly specify it using a guarded concatenation operator which we call finish-guarded concatenation. From the user’s point of view, thisoperator terminates the first machine when the second machine moves into a final state. It chooses
a unique name and uses it to embed a low priority into all transitions of the first machine. A higher
priority is then embedded into the transitions of the second machine that enter into a final state.
The following example yields a machine identical to the example in Section 4.
comment = ’/*’ ( any @comm )* :>> ’*/’;
DEF / comm

IN

0

’/’

1

’*’

’*’ / comm’*’ / comm
3

2

’/’

4

DEF / commAnother guarded operator is left-guarded concatenation, given by the <: compound symbol.
This operator places a higher priority on all transitions of the first machine. This is useful if one
must forcibly separate two lists that contain common elements. For example, one may need totokenize a stream, but first consume leading whitespace.
Ragel also includes a longest-match kleene star operator, given by the ** compound symbol.
This guarded operator embeds a high priority into all transitions of the machine. A lower priority
is then embedded into the leaving transitions. When the kleene star operator makes the epsilontransitions from the final states into the new start state, the lower priority will be transferred tothe epsilon transitions. In cases where following an epsilon transition out of a final state conflicts
with an existing transition out of a final state, the epsilon transition will be dropped.
Other guarded operators are conceivable, such as guards on union that cause one alternative
to take precedence over another. These may be implemented when it is clear they constitute afrequently used operation. In the next section we discuss the explicit specification of state machines
using state charts.

CHAPTER 4. CONTROLLING NONDETERMINISM

4.2.1

32

Entry-Guarded Concatenation

expr :> expr
This operator concatenates two machines, but first assigns a low priority to all transitionsof the first machine and a high priority to the starting transitions of the second machine. This
operator is useful if from the final states of the first machine it is possible to accept the charactersin the entering transitions of the second machine. This operator effectively terminates the firstmachine immediately upon starting the second machine, where otherwise they would be pursued
concurrently. In the following example, entry-guarded concatenation is used to move out of amachine that matches everything at the first sign of an end-of-input marker.
# Leave the catch-all machine on the first character of FIN.
main := any* :> ’FIN’;
DEF

IN

0

’F’

1

’I’

’N’

2

3

Entry-guarded concatenation is equivalent to the following:
expr $(unique_name,0) . expr >(unique_name,1)

4.2.2

Finish-Guarded Concatenation

expr :>> expr
This operator is like the previous operator, except the higher priority is placed on the final
transitions of the second machine. This is useful if one wishes to entertain the possibility ofcontinuing to match the first machine right up until the second machine enters a final state. In
other words it terminates the first machine only when the second accepts. In the following example,
finish-guarded concatenation causes the move out of the machine that matches everything to bedelayed until the full end-of-input marker has been matched.
# Leave the catch-all machine on the last character of FIN.
main := any* :>> ’FIN’;
’F’
DEF
’F’
IN

0

1

’I’
’F’

DEF

2

’N’

3

DEF

Finish-guarded concatenation is equivalent to the following, with one exception. If the rightmachine’s start state is final, the higher priority is also embedded into it as a leaving priority. This
prevents the left machine from persisting via the zero-length string.
expr $(unique_name,0) . expr @(unique_name,1)

CHAPTER 4. CONTROLLING NONDETERMINISM

4.2.3

33

Left-Guarded Concatenation

expr <: expr
This operator places a higher priority on the left expression. It is useful if you want to prefix asequence with another sequence composed of some of the same characters. For example, one canconsume leading whitespace before tokenizing a sequence of whitespace-separated words as in:main := ( ’ ’* >start %fin ) <: ( ’ ’ $ws | [a-z] $alpha )*;’’’a’..’z’ / alpha’ ’ / ws
1

’ ’ / startIN

0’a’..’z’ / fin, alpha2’a’..’z’ / fin, alphaLeft-guarded concatenation is equivalent to the following:
expr $(unique_name,1) . expr >(unique_name,0)4.2.4

Longest-Match Kleene Star

expr**
This version of kleene star puts a higher priority on staying in the machine versus wrapping
around and starting over. The LM kleene star is useful when writing simple tokenizers. Thesemachines are built by applying the longest-match kleene star to an alternation of token patterns,
as in the following.
# Repeat tokens, but make sure to get the longest match.
main := (lower ( lower | digit )* %A |digit+ %B |’ ’)**;
’0’..’9’’’’0’..’9’

1

’’/B
IN

0

’a’..’z’ / B ’0’..’9’, ’a’..’z’’a’..’z’’’/A2

If a regular kleene star were used the machine above would not be able to distinguish between
extending a word and beginning a new one. This operator is equivalent to:( expr $(unique_name,1) %(unique_name,0) )*When the kleene star is applied, transitions that go out of the machine and back into it are

CHAPTER 4. CONTROLLING NONDETERMINISM

34

made. These are assigned a priority of zero by the leaving transition mechanism. This is less thanthe priority of one assigned to the transitions leaving the final states but not leaving the machine.
When these transitions clash on the same character, the transition that stays in the machine takes
precedence. The transition that wraps around is dropped.
Note that this operator does not build a scanner in the traditional sense because there isnever any backtracking. To build a scanner with backtracking use the Longest-Match machine
construction described in Section 6.3.

Chapter 5Interface to Host Program
The Ragel code generator is very flexible. The generated code has no dependencies and can beinserted in any function, perhaps inside a loop if desired. The user is responsible for declaring andinitializing a number of required variables, including the current state and the pointer to the input
stream. These can live in any scope. Control of the input processing loop is also possible: the user
may break out of the processing loop and return to it at any time.
In the case of the C, D, and Go host languages, Ragel is able to generate very fast-running codethat implements state machines as directly executable code. Since very large files strain the hostlanguage compiler, table-based code generation is also supported. In the future we hope to provide
a partitioned, directly executable format that is able to reduce the burden on the host compiler by
splitting large machines across multiple functions.
In the case of Java and Ruby, table-based code generation is the only code style supported. In
the future this may be expanded to include other code styles.
Ragel can be used to parse input in one block, or it can be used to parse input in a sequence ofblocks as it arrives from a file or socket. Parsing the input in a sequence of blocks brings with it a few
responsibilities. If the parser utilizes a scanner, care must be taken to not break the input stream
anywhere but token boundaries. If pointers to the input stream are taken during parsing, caremust be taken to not use a pointer that has been invalidated by movement to a subsequent block.
If the current input data pointer is moved backwards it must not be moved past the beginning of
the current block.
Figure 5.1 shows a simple Ragel program that does not have any actions. The example tests thefirst argument of the program against a number pattern and then prints the machine’s acceptancestatus.

5.1

Variables Used by Ragel

There are a number of variables that Ragel expects the user to declare. At a very minimum the cs,
p and pe variables must be declared. In Go, Java and Ruby code the data variable must also be
declared. If EOF actions are used then the eof variable is required. If stack-based state machine
control flow statements are used then the stack and top variables are required. If a scanner isdeclared then the act, ts and te variables must be declared.
• cs - Current state. This must be an integer and it should persist across invocations of themachine when the data is broken into blocks that are processed independently. This variable
may be modified from outside the execution loop, but not from within.

35

CHAPTER 5. INTERFACE TO HOST PROGRAM

36#include <stdio.h>#include <string.h>%%{machine foo;
write data;}%%
int main( int argc, char **argv ){int cs;if ( argc > 1 ) {char *p = argv[1];char *pe = p + strlen( p );%%{main := [0-9]+ ( ’.’ [0-9]+ )?;write init;
write exec;}%%}
printf("result = %i\n", cs >= foo_first_final );return 0;}

Figure 5.1: A basic Ragel example without any actions.
• p - Data pointer. In C/D code this variable is expected to be a pointer to the character data
to process. It should be initialized to the beginning of the data block on every run of themachine. In Go, Java and Ruby it is used as an offset to data and must be an integer. In
this case it should be initialized to zero on every run of the machine.
• pe - Data end pointer. This should be initialized to p plus the data length on every run ofthe machine. In Go, Java and Ruby code this should be initialized to the data length.
• eof - End of file pointer. This should be set to pe when the buffer block being processed isthe last one, otherwise it should be set to null. In Go, Java and Ruby code -1 must be usedinstead of null. If the EOF event can be known only after the final buffer block has beenprocessed, then it is possible to set p = pe = eof and run the execute block.
• data - This variable is only required in Go, Java and Ruby code. It must be an array
containting the data to process.
• stack - This must be an array of integers. It is used to store integer values representing
states. If the stack must resize dynamically the Pre-push and Post-Pop statements can beused to do this (Sections 5.6 and 5.7).
• top - This must be an integer value and will be used as an offset to stack, giving the next
available spot on the top of the stack.
• act - This must be an integer value. It is a variable sometimes used by scanner code to keep
track of the most recent successful pattern match.
• ts - This must be a pointer to character data. In Go, Java and Ruby code this must be an
integer. See Section 6.3 for more information.

CHAPTER 5. INTERFACE TO HOST PROGRAM

37• te - Also a pointer to character data.

5.2

Alphtype Statement

alphtype unsigned int;
The alphtype statement specifies the alphabet data type that the machine operates on. During
the compilation of the machine, integer literals are expected to be in the range of possible valuesof the alphtype. The default is char for all languages except Go where the default is byte.
C/C++/Objective-C:
char
unsigned char
short
unsigned shortint
unsigned int
longunsigned long
Go:
byte
int8
uint8
int16
uint16
int32
uint32
int64
uint64
rune
Ruby:
char
int

5.3

Java:
char
byte
short
int
D:
char
byte
short
wchar
int
dchar

ubyte
ushortuint

Getkey Statement

getkey fpc->id;
This statement specifies to Ragel how to retrieve the current character from from the pointer tothe current element (p). Any expression that returns a value of the alphabet type may be used. The
getkey statement may be used for looking into element structures or for translating the characterto process. The getkey expression defaults to (*p). In goto-driven machines the getkey expression
may be evaluated more than once per element processed, therefore it should not incur a large cost
nor preclude optimization.

5.4

Access Statement

access fsm->;
The access statement specifies how the generated code should access the machine data thatis persistent across processing buffer blocks. This applies to all variables except p, pe and eof.
This includes cs, top, stack, ts, te and act. The access statement is useful if a machine is to
be encapsulated inside a structure in C code. It can be used to give the name of a pointer to thestructure.

CHAPTER 5. INTERFACE TO HOST PROGRAM

5.5

38

Variable Statement

variable p fsm->p;
The variable statement specifies how to access a specific variable. All of the variables that are
declared by the user and used by Ragel can be changed. This includes p, pe, eof, cs, top, stack,ts, te and act. In Go, Ruby and Java code generation the data variable can also be changed.

5.6

Pre-Push Statement

prepush {/* stack growing code */}The prepush statement allows the user to supply stack management code that is written out
during the generation of fcall, immediately before the current state is pushed to the stack. This
statement can be used to test the number of available spaces and dynamically grow the stack ifnecessary.

5.7

Post-Pop Statement

postpop {/* stack shrinking code */}The postpop statement allows the user to supply stack management code that is written out
during the generation of fret, immediately after the next state is popped from the stack. This
statement can be used to dynamically shrink the stack.

5.8

Write Statement

write <component> [options];
The write statement is used to generate parts of the machine. There are seven componentsthat can be generated by a write statement. These components make up the state machine’s data,
initialization code, execution code, and export definitions. A write statement may appear before amachine is fully defined. This allows one to write out the data first then later define the machine
where it is used. An example of this is shown in Figure 5.2.

5.8.1

Write Data

write data [options];
The write data statement causes Ragel to emit the constant static data needed by the machine.
In table-driven output styles (see Section 5.11) this is a collection of arrays that represent thestates and transitions of the machine. In goto-driven machines much less data is emitted. At thevery minimum a start state name_start is generated. All variables written out in machine data
have both the static and const properties and are prefixed with the name of the machine and an

CHAPTER 5. INTERFACE TO HOST PROGRAM

39#include <stdio.h>%% machine foo;%% write data;
int main( int argc, char **argv ){int cs, res = 0;
if ( argc > 1 ) {char *p = argv[1];%%{main :=[a-z]+
0 @{ res = 1; fbreak; };write init;
write exec noend;}%%}
printf("execute = %i\n", res );return 0;}

Figure 5.2: Use of noend write option and the fbreak statement for processing a string.
underscore. The data can be placed inside a class, inside a function, or it can be defined as global
data.
Two variables are written that may be used to test the state of the machine after a buffer block
has been processed. The name_error variable gives the id of the state that the machine moves
into when it cannot find a valid transition to take. The machine immediately breaks out of theprocessing loop when it finds itself in the error state. The error variable can be compared to thecurrent state to determine if the machine has failed to parse the input. If the machine is complete,that is from every state there is a transition to a proper state on every possible character of thealphabet, then no error state is required and this variable will be set to -1.
The name_first_final variable stores the id of the first final state. All of the machine’s states
are sorted by their final state status before having their ids assigned. Checking if the machine has
accepted its input can then be done by checking if the current state is greater-than or equal to thefirst final state.
Data generation has several options:• noerror• nofinal• noentry• noprefix

5.8.2- Do not generate the integer variable that gives the id of the error state.
- Do not generate the integer variable that gives the id of the first final state.
- Do not generate the integer variables that give the values of the entry points.
- Do not prefix the variable names with the name of the machine.

Write Start, First Final and Error

write start;
write first_final;
write error;
These three write statements provide an alternative means of accessing the start, first_final
and error states. If there are many different machine specifications in one file it is easy to get theprefix for these wrong. This is especially true if the state machine boilerplate is frequently made

CHAPTER 5. INTERFACE TO HOST PROGRAM

40

by a copy-paste-edit process. These write statements allow the problem to be avoided. They canbe used as follows:/* Did parsing succeed? */if ( cs < %%{ write first_final; }%% ) {result = ERR_PARSE_ERROR;
goto fail;
}

5.8.3

Write Init

write init [options];
The write init statement causes Ragel to emit initialization code. This should be executed
once before the machine is started. At a very minimum this sets the current state to the start
state. If other variables are needed by the generated code, such as call stack variables or scanner
management variables, they are also initialized here.
The nocs option to the write init statement will cause ragel to skip intialization of the cs
variable. This is useful if the user wishes to use custom logic to decide which state the specification
should start in.

5.8.4

Write Exec

write exec [options];
The write exec statement causes Ragel to emit the state machine’s execution code. Ragel
expects several variables to be available to this code. At a very minimum, the generated codeneeds access to the current character position p, the ending position pe and the current state cs(though pe can be omitted using the noend write option). The p variable is the cursor that theexecute code will used to traverse the input. The pe variable should be set up to point to one
position past the last valid character in the buffer.
Other variables are needed when certain features are used. For example using the fcall or
fret statements requires stack and top variables to be defined. If a longest-match construction isused, variables for managing backtracking are required.
The write exec statement has one option. The noend option tells Ragel to generate code that
ignores the end position pe. In this case the user must explicitly break out of the processing loop
using fbreak, otherwise the machine will continue to process characters until it moves into theerror state. This option is useful if one wishes to process a null terminated string. Rather thantraverse the string to discover then length before processing the input, the user can break out when
the null character is seen. The example in Figure 5.2 shows the use of the noend write option andthe fbreak statement for processing a string.

5.8.5

Write Exports

write exports;
The export feature can be used to export simple machine definitions. Machine definitions aremarked for export using the export keyword.
export machine_to_export = 0x44;

CHAPTER 5. INTERFACE TO HOST PROGRAM

41

When the write exports statement is used these machines are written out in the generated code.
Defines are used for C and constant integers are used for D, Java and Ruby. See Section 2.1.5 fora description of the import statement.

5.9

Maintaining Pointers to Input Data

In the creation of any parser it is not uncommon to require the collection of the data being parsed.
It is always possible to collect data into a growable buffer as the machine moves over it, however
the copying of data is a somewhat wasteful use of processor cycles. The most efficient way tocollect data from the parser is to set pointers into the input then later reference them. This posesa problem for uses of Ragel where the input data arrives in blocks, such as over a socket or from afile. If a pointer is set in one buffer block but must be used while parsing a following buffer block,some extra consideration to correctness must be made.
The scanner constructions exhibit this problem, requiring the maintenance code described in
Section 6.3. If a longest-match construction has been used somewhere in the machine then it ispossible to take advantage of the required prefix maintenance code in the driver program to ensure
pointers to the input are always valid. If laying down a pointer one can set ts at the same spotor ahead of it. When data is shifted in between loops the user must also shift the pointer. In thisway it is possible to maintain pointers to the input that will always be consistent.
In general, there are two approaches for guaranteeing the consistency of pointers to input data.
The first approach is the one just described; lay down a marker from an action, then later ensurethat the data the marker points to is preserved ahead of the buffer on the next execute invocation.
This approach is good because it allows the parser to decide on the pointer-use boundaries, which
can be arbitrarily complex parsing conditions. A downside is that it requires any pointers that are
set to be corrected in between execute invocations.
The alternative is to find the pointer-use boundaries before invoking the execute routine, thenpass in the data using these boundaries. For example, if the program must perform line-oriented
processing, the user can scan backwards from the end of an input block that has just been readin and process only up to the first found newline. On the next input read, the new data is placed
after the partially read line and processing continues from the beginning of the line. An exampleof line-oriented processing is given in Figure 5.3.

5.10

Specifying the Host Language

The ragel program has a number of options for specifying the host language. The host-language
options are:• -C

for C/C++/Objective-C code (default)• -D

for D code.

• -Z

for Go code.

• -J

for Java code.

• -R

for Ruby code.

• -A

for C# code.

CHAPTER 5. INTERFACE TO HOST PROGRAM

42

int have = 0;
while ( 1 ) {char *p, *pe, *data = buf + have;
int len, space = BUFSIZE - have;if ( space == 0 ) {fprintf(stderr, "BUFFER OUT OF SPACE\n");exit(1);}
len = fread( data, 1, space, stdin );if ( len == 0 )break;/* Find the last newline by searching backwards. */p = buf;
pe = data + len - 1;
while ( *pe != ’\n’ && pe >= buf )pe--;
pe += 1;%% write exec;/* How much is still in the buffer? */have = data + len - pe;if ( have > 0 )memmove( buf, pe, have );if ( len < space )break;}

Figure 5.3: An example of line-oriented processing.

5.11

Choosing a Generated Code Style

There are three styles of code output to choose from. Code style affects the size and speed of thecompiled binary. Changing code style does not require any change to the Ragel program. There
are two table-driven formats and a goto driven format.
In addition to choosing a style to emit, there are various levels of action code reuse to choose
from. The maximum reuse levels (-T0, -F0 and -G0) ensure that no FSM action code is ever
duplicated by encoding each transition’s action list as static data and iterating through the lists on
every transition. This will normally result in a smaller binary. The less action reuse options (-T1,-F1 and -G1) will usually produce faster running code by expanding each transition’s action list
into a single block of code, eliminating the need to iterate through the lists. This duplicates action
code instead of generating the logic necessary for reuse. Consequently the binary will be larger.
However, this tradeoff applies to machines with moderate to dense action lists only. If a machine’s
transitions frequently have less than two actions then the less reuse options will actually produce
both a smaller and a faster running binary due to less action sharing overhead. The best way tochoose the appropriate code style for your application is to perform your own tests.
The table-driven FSM represents the state machine as constant static data. There are tables ofCHAPTER 5. INTERFACE TO HOST PROGRAM

43states, transitions, indices and actions. The current state is stored in a variable. The execution is
simply a loop that looks up the current state, looks up the transition to take, executes any actions
and moves to the target state. In general, the table-driven FSM can handle any machine, producesa smaller binary and requires a less expensive host language compile, but results in slower running
code. Since the table-driven format is the most flexible it is the default code style.
The flat table-driven machine is a table-based machine that is optimized for small alphabets.
Where the regular table machine uses the current character as the key in a binary search for thetransition to take, the flat table machine uses the current character as an index into an array oftransitions. This is faster in general, however is only suitable if the span of possible characters issmall.
The goto-driven FSM represents the state machine using goto and switch statements. The
execution is a flat code block where the transition to take is computed using switch statements anddirectly executable binary searches. In general, the goto FSM produces faster code but results ina larger binary and a more expensive host language compile.
The goto-driven format has an additional action reuse level (-G2) that writes actions directly
into the state transitioning logic rather than putting all the actions together into a single switch.
Generally this produces faster running code because it allows the machine to encode the current
state using the processor’s instruction pointer. Again, sparse machines may actually compile tosmaller binaries when -G2 is used due to less state and action management overhead. For manyparsing applications -G2 is the preferred output format.

-T0
-T1
-F0
-F1
-G0
-G1
-G2

Code Output Style Options
binary search table-driven
C/D/Java/Ruby/C#/Go
binary search, expanded actions
C/D/Ruby/C#/Go
flat table-driven
C/D/Ruby/C#/Go
flat table, expanded actions
C/D/Ruby/C#/Go
goto-driven
C/D/C#/Go
goto, expanded actions
C/D/C#/Go
goto, in-place actions
C/D/GoChapter 6

Beyond the Basic Model
6.1

Parser Modularization

It is possible to use Ragel’s machine construction and action embedding operators to specify anentire parser using a single regular expression. In many cases this is the desired way to specify aparser in Ragel. However, in some scenarios the language to parse may be so large that it is difficult
to think about it as a single regular expression. It may also shift between distinct parsing strategies,in which case modularization into several coherent blocks of the language may be appropriate.
It may also be the case that patterns that compile to a large number of states must be used ina number of different contexts and referencing them in each context results in a very large statemachine. In this case, an ability to reuse parsers would reduce code size.
To address this, distinct regular expressions may be instantiated and linked together by meansof a jumping and calling mechanism. This mechanism is analogous to the jumping to and calling ofprocessor instructions. A jump command, given in action code, causes control to be immediately
passed to another portion of the machine by way of setting the current state variable. A callcommand causes the target state of the current transition to be pushed to a state stack before
control is transferred. Later on, the original location may be returned to with a return statement.
In the following example, distinct state machines are used to handle the parsing of two types ofheaders.
action return { fret; }action call_date { fcall date; }action call_name { fcall name; }# A parser for date strings.
date := [0-9][0-9] ’/’[0-9][0-9] ’/’[0-9][0-9][0-9][0-9] ’\n’ @return;# A parser for name strings.
name := ( [a-zA-Z]+ | ’ ’ )** ’\n’ @return;# The main parser.
headers =( ’from’ | ’to’ ) ’:’ @call_name |( ’departed’ | ’arrived’ ) ’:’ @call_date;44

CHAPTER 6. BEYOND THE BASIC MODEL

45

main := headers*;

Calling and jumping should be used carefully as they are operations that take one out of thedomain of regular languages. A machine that contains a call or jump statement in one of its actions
should be used as an argument to a machine construction operator only with considerable care.
Since DFA transitions may actually represent several NFA transitions, a call or jump embedded
in one machine can inadvertently terminate another machine that it shares prefixes with. Despite
this danger, theses statements have proven useful for tying together sub-parsers of a language intoa parser for the full language, especially for the purpose of modularizing code and reducing thenumber of states when the machine contains frequently recurring patterns.
Section 3.4 describes the jump and call statements that are used to transfer control. Thesestatements make use of two variables that must be declared by the user, stack and top. The
stack variable must be an array of integers and top must be a single integer, which will point
to the next available space in stack. Sections 5.6 and 5.7 describe the Pre-Push and Post-Pop
statements which can be used to implement a dynamically resizable array.

6.2

Referencing Names

This section describes how to reference names in epsilon transitions (Section 6.4) and actionbased control-flow statements such as fgoto. There is a hierarchy of names implied in a Ragelspecification. At the top level are the machine instantiations. Beneath the instantiations are labels
and references to machine definitions. Beneath those are more labels and references to definitions,and so on.
Any name reference may contain multiple components separated with the :: compound symbol.
The search for the first component of a name reference is rooted at the join expression that theepsilon transition or action embedding is contained in. If the name reference is not contained in a
join, the search is rooted at the machine definition that the epsilon transition or action embedding
is contained in. Each component after the first is searched for beginning at the location in thename tree that the previous reference component refers to.
In the case of action-based references, if the action is embedded more than once, the local searchis performed for each embedding and the result is the union of all the searches. If no result is foundfor action-based references then the search is repeated at the root of the name tree. Any actionbased name search may be forced into a strictly global search by prefixing the name reference with::.
The final component of the name reference must resolve to a unique entry point. If a name isunique in the entire name tree it can be referenced as is. If it is not unique it can be specified byqualifying it with names above it in the name tree. However, it can always be renamed.

6.3

Scanners

Scanners are very much intertwined with regular-languages and their corresponding processors. For
this reason Ragel supports the definition of scanners. The generated code will repeatedly attempt
to match patterns from a list, favouring longer patterns over shorter patterns. In the case of equallength matches, the generated code will favour patterns that appear ahead of others. When ascanner makes a match it executes the user code associated with the match, consumes the input
then resumes scanning.
<machine_name> := |*CHAPTER 6. BEYOND THE BASIC MODEL

46

pattern1 => action1;
pattern2 => action2;...
*|;
On the surface, Ragel scanners are similar to those defined by Lex. Though there is a key
distinguishing feature: patterns may be arbitrary Ragel expressions and can therefore containembedded code. With a Ragel-based scanner the user need not wait until the end of a pattern
before user code can be executed.
Scanners can be used to process sub-languages, as well as for tokenizing programming languages.
In the following example a scanner is used to tokenize the contents of a header field.
word = [a-z]+;
head_name = ’Header’;header := |*
word;
’ ’;
’\n’ => { fret; };*|;main := ( head_name ’:’ @{ fcall header; } )*;The scanner construction has a purpose similar to the longest-match kleene star operator **.
The key difference is that a scanner is able to backtrack to match a previously matched shorter
string when the pursuit of a longer string fails. For this reason the scanner construction operator
is not a pure state machine construction operator. It relies on several variables that enable it
to backtrack and make pointers to the matched input text available to the user. For this reason
scanners must be immediately instantiated. They cannot be defined inline or referenced by anotherexpression. Scanners must be jumped to or called.
Scanners rely on the ts, te and act variables to be present so that they can backtrack andmake pointers to the matched text available to the user. If input is processed using multiple calls
to the execute code then the user must ensure that when a token is only partially matched thatthe prefix is preserved on the subsequent invocation of the execute code.
The ts variable must be defined as a pointer to the input data. It is used for recording where
the current token match begins. This variable may be used in action code for retrieving the text ofthe current match. Ragel ensures that in between tokens and outside of the longest-match machinesthat this pointer is set to null. In between calls to the execute code the user must check if ts is
set and if so, ensure that the data it points to is preserved ahead of the next buffer block. This is
described in more detail below.
The te variable must also be defined as a pointer to the input data. It is used for recording
where a match ends and where scanning of the next token should begin. This can also be used inaction code for retrieving the text of the current match.
The act variable must be defined as an integer type. It is used for recording the identity of thelast pattern matched when the scanner must go past a matched pattern in an attempt to make alonger match. If the longer match fails it may need to consult the act variable. In some cases, useof the act variable can be avoided because the value of the current state is enough information todetermine which token to accept, however in other cases this is not enough and so the act variable
is used.

CHAPTER 6. BEYOND THE BASIC MODELa)

A stream "of characters" to be scanned.
|
|
|
p
ts
pe

b)

"of characters" to be scanned.
|
|
|
ts
p
pe

47

Figure 6.1: Following an invocation of the execute code there may be a partially matched token(a). The data of the partially matched token must be preserved ahead of the new data on the next
invocation (b).
When the longest-match operator is in use, the user’s driver code must take on some buffer
management functions. The following algorithm gives an overview of the steps that should betaken to properly use the longest-match operator.
• Read a block of input data.
• Run the execute code.
• If ts is set, the execute code will expect the incomplete token to be preserved ahead of thebuffer on the next invocation of the execute code.
– Shift the data beginning at ts and ending at pe to the beginning of the input buffer.
– Reset ts to the beginning of the buffer.
– Shift te by the distance from the old value of ts to the new value. The te variable may
or may not be valid. There is no way to know if it holds a meaningful value because it
is not kept at null when it is not in use. It can be shifted regardless.
• Read another block of data into the buffer, immediately following any preserved data.
• Run the scanner on the new data.
Figure 6.1 shows the required handling of an input stream in which a token is broken by the
input block boundaries. After processing up to and including the “t” of “characters”, the prefix ofthe string token must be retained and processing should resume at the “e” on the next iteration of
the execute code.
If one uses a large input buffer for collecting input then the number of times the shifting must
be done will be small. Furthermore, if one takes care not to define tokens that are allowed to bevery long and instead processes these items using pure state machines or sub-scanners, then only asmall amount of data will ever need to be shifted.
Since scanners attempt to make the longest possible match of input, patterns such as identifiers
require one character of lookahead in order to trigger a match. In the case of the last token in the
input stream the user must ensure that the eof variable is set so that the final token is flushed out.
An example scanner processing loop is given in Figure 6.2.

6.4

State Charts

In addition to supporting the construction of state machines using regular languages, Ragel provides
a way to manually specify state machines using state charts. The comma operator combines
machines together without any implied transitions. The user can then manually link machines by

CHAPTER 6. BEYOND THE BASIC MODEL48

int have = 0;bool done = false;
while ( !done ) {/* How much space is in the buffer? */int space = BUFSIZE - have;if ( space == 0 ) {/* Buffer is full. */cerr << "TOKEN TOO BIG" << endl;
exit(1);}/* Read in a block after any data we already have. */char *p = inbuf + have;cin.read( p, space );int len = cin.gcount();
char *pe = p + len;char *eof = 0;/* If no data was read indicate EOF. */if ( len == 0 ) {eof = pe;done = true;}%% write exec;
if ( cs == Scanner_error ) {/* Machine failed before finding a token. */cerr << "PARSE ERROR" << endl;
exit(1);}
if ( ts == 0 )have = 0;else {/* There is a prefix to preserve, shift it over. */have = pe - ts;
memmove( inbuf, ts, have );te = inbuf + (te-ts);
ts = inbuf;}}

Figure 6.2: A processing loop for a scanner.
specifying epsilon transitions with the -> operator. Epsilon transitions are drawn between the final
states of a machine and entry points defined by labels. This makes it possible to build machines
using the explicit state-chart method while making minimal changes to the Ragel language.
An interesting feature of Ragel’s state chart construction method is that it can be mixed freely
with regular expression constructions. A state chart may be referenced from within a regularexpression, or a regular expression may be used in the definition of a state chart transition.

CHAPTER 6. BEYOND THE BASIC MODEL

6.4.1

49

Join

expr , expr , ...
Join a list of machines together without drawing any transitions, without setting up a start
state, and without designating any final states. Transitions between the machines may be specified
using labels and epsilon transitions. The start state must be explicity specified with the “start”
label. Final states may be specified with an epsilon transition to the implicitly created “final” state.
The join operation allows one to build machines using a state chart model.

6.4.2

Label

label: expr
Attaches a label to an expression. Labels can be used as the target of epsilon transitions andexplicit control transfer statements such as fgoto and fnext in action code.

6.4.3

Epsilonexpr -> label
Draws an epsilon transition to the state defined by label. Epsilon transitions are made deterministic when join operators are evaluated. Epsilon transitions that are not in a join operation are
made deterministic when the machine definition that contains the epsilon is complete. See Section
6.2 for information on referencing labels.

6.4.4

Simplifying State Charts

There are two benefits to providing state charts in Ragel. The first is that it allows us to takea state chart with a full listing of states and transitions and simplify it in selective places using
regular expressions.
The state chart method of specifying parsers is very common. It is an effective programming
technique for producing robust code. The key disadvantage becomes clear when one attempts tocomprehend a large parser specified in this way. These programs usually require many lines, causing
logic to be spread out over large distances in the source file. Remembering the function of a large
number of states can be difficult and organizing the parser in a sensible way requires discipline
because branches and repetition present many file layout options. This kind of programming takes
a specification with inherent structure such as looping, alternation and concatenation and expresses
it in a flat form.
If we could take an isolated component of a manually programmed state chart, that is, a subsetof states that has only one entry point, and implement it using regular language operators then we
could eliminate all the explicit naming of the states contained in it. By eliminating explicitly named
states and replacing them with higher-level specifications we simplify a state machine specification.
For example, sometimes chains of states are needed, with only a small number of possible characters appearing along the chain. These can easily be replaced with a concatenation of characters.
Sometimes a group of common states implement a loop back to another single portion of the machine. Rather than manually duplicate all the transitions that loop back, we may be able to express
the loop using a kleene star operator.
Ragel allows one to take this state map simplification approach. We can build state machines
using a state map model and implement portions of the state map using regular languages. In place

CHAPTER 6. BEYOND THE BASIC MODEL

50

of any transition in the state machine, entire sub-machines can be given. These can encapsulate
functionality defined elsewhere. An important aspect of the Ragel approach is that when we wrap
up a collection of states using a regular expression we do not lose access to the states and transitions.
We can still execute code on the transitions that we have encapsulated.

6.4.5

Dropping Down One Level of Abstraction

The second benefit of incorporating state charts into Ragel is that it permits us to bypass theregular language abstraction if we need to. Ragel’s action embedding operators are sometimes
insufficient for expressing certain parsing tasks. In the same way that is useful for C language
programmers to drop down to assembly language programming using embedded assembler, it issometimes useful for the Ragel programmer to drop down to programming with state charts.
In the following example, we wish to buffer the characters of an XML CDATA sequence. The
sequence is terminated by the string ]]>. The challenge in our application is that we do not wish
the terminating characters to be buffered. An expression of the form any* @buffer :>> ’]]>’
will not work because the buffer will always contain the characters ]] on the end. Instead, what
we need is to delay the buffering of ] characters until a time when we abandon the terminating
sequence and go back into the main loop. There is no easy way to express this using Ragel’s
regular expression and action embedding operators, and so an ability to drop down to the statechart method is useful.
action bchar { buff( fpc ); }# Buffer the current character.
action bbrack1 { buff( "]" ); }action bbrack2 { buff( "]]" ); }CDATA_body =start: (’]’ -> one |(any-’]’) @bchar ->start),
one: (’]’ -> two |[^\]] @bbrack1 @bchar ->start),
two: (’>’ -> final |’]’ @bbrack1 -> two |[^>\]] @bbrack2 @bchar ->start);
DEF / bchar’]’ / bbrack1

’]’
1
IN

0

’]’

DEF / bbrack1, bchar
DEF / bbrack2, bchar

6.5

2

’>’

3

Semantic Conditions

Many communication protocols contain variable-length fields, where the length of the field is given
ahead of the field as a value. This problem cannot be expressed using regular languages because of

CHAPTER 6. BEYOND THE BASIC MODEL51

its context-dependent nature. The prevalence of variable-length fields in communication protocols
motivated us to introduce semantic conditions into the Ragel language.
A semantic condition is a block of user code that is interpreted as an expression and evaluated
immediately before a transition is taken. If the code returns a value of true, the transition may betaken. We can now embed code that extracts the length of a field, then proceed to match n data
values.
action rec_num { i = 0; n = getnumber(); }action test_len { i++ < n }data_fields = (’d’[0-9]+ %rec_num’:’( [a-z] when test_len )*)**;97..122(test_len)

IN

0

100

100(!test_len)
1
48..57

48..5758 / rec_num

3

2

The Ragel implementation of semantic conditions does not force us to give up the compositional
property of Ragel definitions. For example, a machine that tests the length of a field using conditions
can be unioned with another machine that accepts some of the same strings, without the twomachines interfering with one another. The user need not be concerned about whether or not theresult of the semantic condition will affect the matching of the second machine.
To see this, first consider that when a user associates a condition with an existing transition,
the transition’s label is translated from the base character to its corresponding value in the spacethat represents “condition c true”. Should the determinization process combine a state that has
a conditional transition with another state that has a transition on the same input character but
without a condition, then the condition-less transition first has its label translated into two values,
one to its corresponding value in the space that represents “condition c true” and another to its
corresponding value in the space that represents “condition c false”. It is then safe to combine thetwo transitions. This is shown in the following example. Two intersecting patterns are unioned,
one with a condition and one without. The condition embedded in the first pattern does not affectthe second pattern.
action test_len { i++ < n }action one { /* accept pattern one */ }action two { /* accept pattern two */ }patterns =( [a-z] when test_len )+ %one |[a-z][a-z0-9]* %two;
main := patterns ’\n’;

CHAPTER 6. BEYOND THE BASIC MODEL

52
48..57, 97..122

IN

97..122(!test_len)
0

97..122(test_len)

97..122(test_len)

48..57, 97..122(!test_len)

1
10 / one, two

10 / two
2

3

There are many more potential uses for semantic conditions. The user is free to use arbitrarycode and may therefore perform actions such as looking up names in dictionaries, validating input
using external parsing mechanisms or performing checks on the semantic structure of input seen so
far. In the next section we describe how Ragel accommodates several common parser engineering
problems.

Note: The semantic condition feature works only with alphabet types that are smaller in widththan the long type. To implement semantic conditions Ragel needs to be able to allocate characters from the alphabet space. Ragel uses these allocated characters to express ”character C withcondition P true” or ”C with P false.” Since internally Ragel uses longs to store characters there
is no room left in the alphabet space unless an alphabet type smaller than long is used.

6.6

Implementing Lookahead

There are a few strategies for implementing lookahead in Ragel programs. Leaving actions, which
are described in Section 3.1.4, can be used as a form of lookahead. Ragel also provides the fhold
directive which can be used in actions to prevent the machine from advancing over the current character. It is also possible to manually adjust the current character position by shifting it backwards
using fexec, however when this is done, care must be taken not to overstep the beginning of thecurrent buffer block. In both the use of fhold and fexec the user must be cautious of combining
the resulting machine with another in such a way that the transition on which the current positionis adjusted is not combined with a transition from the other machine.

6.7

Parsing Recursive Language Structures

In general Ragel cannot handle recursive structures because the grammar is interpreted as a regular
language. However, depending on what needs to be parsed it is sometimes practical to implement
the recursive parts using manual coding techniques. This often works in cases where the recursive
structures are simple and easy to recognize, such as in the balancing of parentheses
One approach to parsing recursive structures is to use actions that increment and decrement
counters or otherwise recognize the entry to and exit from recursive structures and then jump to
the appropriate machine defnition using fcall and fret. Alternatively, semantic conditions canbe used to test counter variables.
A more traditional approach is to call a separate parsing function (expressed in the host language) when a recursive structure is entered, then later return when the end is recognized.