neulab/tldr huggingface dataset:
(If your environment supports function prototypes, then it will be "int yylex( void )".)  This definition may be changed by defining the "YY_DECL" macro.  For example, you could use:
#define YY_DECL float lexscan( a, b ) float a, b;
to give the scanning routine the name lexscan, returning a float, and taking two floats as arguments.  Note that if you give arguments to the scanning routine using a K&R-style/non-prototyped function declaration, you must terminate the definition with a semi-colon (;).
Whenever yylex() is called, it scans tokens from the global input file yyin (which defaults to stdin).  It continues until it either reaches an end-of-file (at which point it returns the value 0) or one of its actions executes a return statement.
If the scanner reaches an end-of-file, subsequent calls are undefined unless either yyin is pointed at a new input file (in which case scanning continues from that file), or yyrestart() is called.  yyrestart() takes one argument, a FILE * pointer (which can be nil, if you've set up YY_INPUT to scan from a source other than yyin), and initializes yyin for scanning from that file.  Essentially there is no difference between just assigning yyin to a new input file or using yyrestart() to do so; the latter is available for compatibility with previous versions of flex, and because it can be used to switch input files in the middle of scanning. It can also be used to throw away the current input buffer, by calling it with an argument of yyin; but better is to use YY_FLUSH_BUFFER (see above).  Note that yyrestart() does not reset the start condition to INITIAL (see Start Conditions, below).
If yylex() stops scanning due to executing a return statement in one of the actions, the scanner may then be called again and it will resume scanning where it left off.
By default (and for purposes of efficiency), the scanner uses block-reads rather than simple getc() calls to read characters from yyin.  The nature of how it gets its input can be controlled by defining the YY_INPUT macro.  YY_INPUT's calling sequence is "YY_INPUT(buf,result,max_size)". Its action is to place up to max_size characters in the character array buf and return in the integer variable result either the number of characters read or the constant YY_NULL (0 on Unix systems) to indicate EOF.  The default YY_INPUT reads from the global file-pointer "yyin".
A sample definition of YY_INPUT (in the definitions section of the input file):
%{ #define YY_INPUT(buf,result,max_size) \ { \ int c = getchar(); \ result = (c == EOF) ? YY_NULL : (buf[0] = c, 1); \ } %}
This definition will change the input processing to occur one character at a time.
When the scanner receives an end-of-file indication from YY_INPUT, it then checks the yywrap() function.  If yywrap() returns false (zero), then it is assumed that the function has gone ahead and set up yyin to point to another input file, and scanning continues.  If it returns true (non-zero), then the scanner terminates, returning 0 to its caller.  Note that in either case, the start condition remains unchanged; it does not revert to INITIAL.
If you do not supply your own version of yywrap(), then you must either use %option noyywrap (in which case the scanner behaves as though yywrap() returned 1), or you must link with -lfl to obtain the default version of the routine, which always returns 1.
Three routines are available for scanning from in-memory buffers rather than files: yy_scan_string(), yy_scan_bytes(), and yy_scan_buffer().  See the discussion of them below in the section Multiple Input Buffers.
The scanner writes its ECHO output to the yyout global (default, stdout), which may be redefined by the user simply by assigning it to some other FILE pointer.
flex provides a mechanism for conditionally activating rules.  Any rule whose pattern is prefixed with "<sc>" will only be active when the scanner is in the start condition named "sc".  For example,
<STRING>[^"]*        { /* eat up the string body ... */ ... }
will be active only when the scanner is in the "STRING" start condition, and
<INITIAL,STRING,QUOTE>\.        { /* handle an escape ... */ ... }
will be active only when the current start condition is either "INITIAL", "STRING", or "QUOTE".
Start conditions are declared in the definitions (first) section of the input using unindented lines beginning with either %s or %x followed by a list of names.  The former declares inclusive start conditions, the latter exclusive start conditions.  A start condition is activated using the BEGIN action.  Until the next BEGIN action is executed, rules with the given start condition will be active and rules with other start conditions will be inactive.  If the start condition is inclusive, then rules with no start conditions at all will also be active.  If it is exclusive, then only rules qualified with the start condition will be active.  A set of rules contingent on the same exclusive start condition describe a scanner which is independent of any of the other rules in the flex input.  Because of this, exclusive start conditions make it easy to specify "mini-scanners" which scan portions of the input that are syntactically different from the rest (e.g., comments).
If the distinction between inclusive and exclusive start conditions is still a little vague, here's a simple example illustrating the connection between the two.  The set of rules:
%s example %%
<example>foo   do_something();
bar            something_else();
%x example %%
<INITIAL,example>bar    something_else();
Without the <INITIAL,example> qualifier, the bar pattern in the second example wouldn't be active (i.e., couldn't match) when in start condition example.  If we just used <example> to qualify bar, though, then it would only be active in example and not in INITIAL, while in the first example it's active in both, because in the first example the example startion condition is an inclusive (%s) start condition.
Also note that the special start-condition specifier <*> matches every start condition.  Thus, the above example could also have been written;
<*>bar    something_else();
The default rule (to ECHO any unmatched character) remains active in start conditions.  It is equivalent to:
<*>.|\n     ECHO;
BEGIN(0) returns to the original state where only the rules with no start conditions are active.  This state can also be referred to as the start- condition "INITIAL", so BEGIN(INITIAL) is equivalent to BEGIN(0).  (The parentheses around the start condition name are not required but are considered good style.)
BEGIN actions can also be given as indented code at the beginning of the rules section.  For example, the following will cause the scanner to enter the "SPECIAL" start condition whenever yylex() is called and the global variable enter_special is true:
int enter_special;
%x SPECIAL %% if ( enter_special ) BEGIN(SPECIAL);
<SPECIAL>blahblahblah ...more rules follow...
To illustrate the uses of start conditions, here is a scanner which provides two different interpretations of a string like "123.456".  By default it will treat it as three tokens, the integer "123", a dot ('.'), and the integer "456".  But if the string is preceded earlier in the line by the string "expect-floats" it will treat it as a single token, the floating-point number 123.456:
%{ #include <math.h> %} %s expect
%% expect-floats        BEGIN(expect);
<expect>[0-9]+"."[0-9]+      { printf( "found a float, = %f\n", atof( yytext ) ); } <expect>\n           { /* that's the end of the line, so * we need another "expect-number" * before we'll recognize any more * numbers */ BEGIN(INITIAL); }
[0-9]+      { printf( "found an integer, = %d\n", atoi( yytext ) ); }
"."         printf( "found a dot\n" );
Here is a scanner which recognizes (and discards) C comments while maintaining a count of the current input line.
%x comment %% int line_num = 1;
"/*"         BEGIN(comment);
<comment>[^*\n]*        /* eat anything that's not a '*' */ <comment>"*"+[^*/\n]*   /* eat up '*'s not followed by '/'s */ <comment>\n             ++line_num; <comment>"*"+"/"        BEGIN(INITIAL);
This scanner goes to a bit of trouble to match as much text as possible with each rule.  In general, when attempting to write a high-speed scanner try to match as much possible in each rule, as it's a big win.
Note that start-conditions names are really integer values and can be stored as such.  Thus, the above could be extended in the following fashion:
%x comment foo %% int line_num = 1; int comment_caller;
"/*"         { comment_caller = INITIAL; BEGIN(comment); }
<foo>"/*"    { comment_caller = foo; BEGIN(comment); }
<comment>[^*\n]*        /* eat anything that's not a '*' */ <comment>"*"+[^*/\n]*   /* eat up '*'s not followed by '/'s */ <comment>\n             ++line_num; <comment>"*"+"/"        BEGIN(comment_caller);
Furthermore, you can access the current start condition using the integer-valued YY_START macro.  For example, the above assignments to comment_caller could instead be written
comment_caller = YY_START;
Flex provides YYSTATE as an alias for YY_START (since that is what's used by AT&T lex).
Note that start conditions do not have their own name-space; %s's and %x's declare names in the same fashion as #define's.
Finally, here's an example of how to match C-style quoted strings using exclusive start conditions, including expanded escape sequences (but not including checking for a string that's too long):
%x str
%% char string_buf[MAX_STR_CONST]; char *string_buf_ptr;
\"      string_buf_ptr = string_buf; BEGIN(str);
<str>\"        { /* saw closing quote - all done */ BEGIN(INITIAL); *string_buf_ptr = '\0'; /* return string constant token type and * value to parser */ }
<str>\n        { /* error - unterminated string constant */ /* generate error message */ }
<str>\\[0-7]{1,3} { /* octal escape sequence */ int result;
(void) sscanf( yytext + 1, "%o", &result );
if ( result > 0xff ) /* error, constant is out-of-bounds */
*string_buf_ptr++ = result; }
<str>\\[0-9]+ { /* generate error - bad escape sequence; something * like '\48' or '\0777777' */ }
<str>\\n  *string_buf_ptr++ = '\n'; <str>\\t  *string_buf_ptr++ = '\t'; <str>\\r  *string_buf_ptr++ = '\r'; <str>\\b  *string_buf_ptr++ = '\b'; <str>\\f  *string_buf_ptr++ = '\f';
<str>\\(.|\n)  *string_buf_ptr++ = yytext[1];
<str>[^\\\n\"]+        { char *yptr = yytext;
while ( *yptr ) *string_buf_ptr++ = *yptr++; }
Often, such as in some of the examples above, you wind up writing a whole bunch of rules all preceded by the same start condition(s).  Flex makes this a little easier and cleaner by introducing a notion of start condition scope.  A start condition scope is begun with:
<SCs>{
where SCs is a list of one or more start conditions.  Inside the start condition scope, every rule automatically has the prefix <SCs> applied to it, until a '}' which matches the initial '{'.  So, for example,
<ESC>{ "\\n"   return '\n'; "\\r"   return '\r'; "\\f"   return '\f'; "\\0"   return '\0'; }
<ESC>"\\n"  return '\n'; <ESC>"\\r"  return '\r'; <ESC>"\\f"  return '\f'; <ESC>"\\0"  return '\0';
Start condition scopes may be nested.
Three routines are available for manipulating stacks of start conditions:
void yy_push_state(int new_state) pushes the current start condition onto the top of the start condition stack and switches to new_state as though you had used BEGIN new_state (recall that start condition names are also integers).
void yy_pop_state() pops the top of the stack and switches to it via BEGIN.
int yy_top_state() returns the top of the stack without altering the stack's contents.
The start condition stack grows dynamically and so has no built-in size limitation.  If memory is exhausted, program execution aborts.
To use start condition stacks, your scanner must include a %option stack directive (see Options below).
Some scanners (such as those which support "include" files) require reading from several input streams.  As flex scanners do a large amount of buffering, one cannot control where the next input will be read from by simply writing a YY_INPUT which is sensitive to the scanning context. YY_INPUT is only called when the scanner reaches the end of its buffer, which may be a long time after scanning a statement such as an "include" which requires switching the input source.
To negotiate these sorts of problems, flex provides a mechanism for creating and switching between multiple input buffers.  An input buffer is created by using:
YY_BUFFER_STATE yy_create_buffer( FILE *file, int size )
which takes a FILE pointer and a size and creates a buffer associated with the given file and large enough to hold size characters (when in doubt, use YY_BUF_SIZE for the size).  It returns a YY_BUFFER_STATE handle, which may then be passed to other routines (see below).  The YY_BUFFER_STATE type is a pointer to an opaque struct yy_buffer_state structure, so you may safely initialize YY_BUFFER_STATE variables to ((YY_BUFFER_STATE) 0) if you wish, and also refer to the opaque structure in order to correctly declare input buffers in source files other than that of your scanner.  Note that the FILE pointer in the call to yy_create_buffer is only used as the value of yyin seen by YY_INPUT; if you redefine YY_INPUT so it no longer uses yyin, then you can safely pass a nil FILE pointer to yy_create_buffer.  You select a particular buffer to scan from using:
void yy_switch_to_buffer( YY_BUFFER_STATE new_buffer )
switches the scanner's input buffer so subsequent tokens will come from new_buffer.  Note that yy_switch_to_buffer() may be used by yywrap() to set things up for continued scanning, instead of opening a new file and pointing yyin at it.  Note also that switching input sources via either yy_switch_to_buffer() or yywrap() does not change the start condition.
void yy_delete_buffer( YY_BUFFER_STATE buffer )
is used to reclaim the storage associated with a buffer.  ( buffer can be nil, in which case the routine does nothing.)  You can also clear the current contents of a buffer using:
void yy_flush_buffer( YY_BUFFER_STATE buffer )
This function discards the buffer's contents, so the next time the scanner attempts to match a token from the buffer, it will first fill the buffer anew using YY_INPUT.
yy_new_buffer() is an alias for yy_create_buffer(), provided for compatibility with the C++ use of new and delete for creating and destroying dynamic objects.
Finally, the YY_CURRENT_BUFFER macro returns a YY_BUFFER_STATE handle to the current buffer.
Here is an example of using these features for writing a scanner which expands include files (the <<EOF>> feature is discussed below):
/* the "incl" state is used for picking up the name * of an include file */ %x incl
%{ #define MAX_INCLUDE_DEPTH 10 YY_BUFFER_STATE include_stack[MAX_INCLUDE_DEPTH]; int include_stack_ptr = 0; %}
%% include             BEGIN(incl);
[a-z]+              ECHO; [^a-z\n]*\n?        ECHO;
<incl>[ \t]*      /* eat the whitespace */ <incl>[^ \t\n]+   { /* got the include file name */ if ( include_stack_ptr >= MAX_INCLUDE_DEPTH ) { fprintf( stderr, "Includes nested too deeply" ); exit( 1 ); }
include_stack[include_stack_ptr++] = YY_CURRENT_BUFFER;
yyin = fopen( yytext, "r" );
if ( ! yyin ) error( ... );
yy_switch_to_buffer( yy_create_buffer( yyin, YY_BUF_SIZE ) );
BEGIN(INITIAL); }
<<EOF>> { if ( --include_stack_ptr < 0 ) { yyterminate(); }
else { yy_delete_buffer( YY_CURRENT_BUFFER ); yy_switch_to_buffer( include_stack[include_stack_ptr] ); } }
Three routines are available for setting up input buffers for scanning in-memory strings instead of files.  All of them create a new input buffer for scanning the string, and return a corresponding YY_BUFFER_STATE handle (which you should delete with yy_delete_buffer() when done with it).  They also switch to the new buffer using yy_switch_to_buffer(), so the next call to yylex() will start scanning the string.
yy_scan_string(const char *str) scans a NUL-terminated string.
yy_scan_bytes(const char *bytes, int len) scans len bytes (including possibly NUL's) starting at location bytes.
Note that both of these functions create and scan a copy of the string or bytes.  (This may be desirable, since yylex() modifies the contents of the buffer it is scanning.)  You can avoid the copy by using:
yy_scan_buffer(char *base, yy_size_t size) which scans in place the buffer starting at base, consisting of size bytes, the last two bytes of which must be YY_END_OF_BUFFER_CHAR (ASCII NUL).  These last two bytes are not scanned; thus, scanning consists of base[0] through base[size-2], inclusive.
If you fail to set up base in this manner (i.e., forget the final two YY_END_OF_BUFFER_CHAR bytes), then yy_scan_buffer() returns a nil pointer instead of creating a new input buffer.
The type yy_size_t is an integral type to which you can cast an integer expression reflecting the size of the buffer.
END-OF-FILE RULES The special rule "<<EOF>>" indicates actions which are to be taken when an end-of-file is encountered and yywrap() returns non-zero (i.e., indicates no further files to process).  The action must finish by doing one of four things:
-      assigning yyin to a new input file (in previous versions of flex, after doing the assignment you had to call the special action YY_NEW_FILE; this is no longer necessary);
-      executing a return statement;
-      executing the special yyterminate() action;
-      or, switching to a new buffer using yy_switch_to_buffer() as shown in the example above.
<<EOF>> rules may not be used with other patterns; they may only be qualified with a list of start conditions.  If an unqualified <<EOF>> rule is given, it applies to all start conditions which do not already have <<EOF>> actions.  To specify an <<EOF>> rule for only the initial start condition, use
<INITIAL><<EOF>>
These rules are useful for catching things like unclosed comments.  An example:
%x quote %%
...other rules for dealing with quotes...
<quote><<EOF>>   { error( "unterminated quote" ); yyterminate(); } <<EOF>>  { if ( *++filelist ) yyin = fopen( *filelist, "r" ); else yyterminate(); }
The macro YY_USER_ACTION can be defined to provide an action which is always executed prior to the matched rule's action.  For example, it could be #define'd to call a routine to convert yytext to lower-case. When YY_USER_ACTION is invoked, the variable yy_act gives the number of the matched rule (rules are numbered starting with 1).  Suppose you want to profile how often each of your rules is matched.  The following would do the trick:
#define YY_USER_ACTION ++ctr[yy_act]
where ctr is an array to hold the counts for the different rules.  Note that the macro YY_NUM_RULES gives the total number of rules (including the default rule, even if you use -s), so a correct declaration for ctr is:
int ctr[YY_NUM_RULES];
The macro YY_USER_INIT may be defined to provide an action which is always executed before the first scan (and before the scanner's internal initializations are done).  For example, it could be used to call a routine to read in a data table or open a logging file.
The macro yy_set_interactive(is_interactive) can be used to control whether the current buffer is considered interactive.  An interactive buffer is processed more slowly, but must be used when the scanner's input source is indeed interactive to avoid problems due to waiting to fill buffers (see the discussion of the -I flag below).  A non-zero value in the macro invocation marks the buffer as interactive, a zero value as non-interactive.  Note that use of this macro overrides %option always- interactive or %option never-interactive (see Options below). yy_set_interactive() must be invoked prior to beginning to scan the buffer that is (or is not) to be considered interactive.
The macro yy_set_bol(at_bol) can be used to control whether the current buffer's scanning context for the next token match is done as though at the beginning of a line.  A non-zero macro argument makes rules anchored with '^' active, while a zero argument makes '^' rules inactive.
The macro YY_AT_BOL() returns true if the next token scanned from the current buffer will have '^' rules active, false otherwise.
In the generated scanner, the actions are all gathered in one large switch statement and separated using YY_BREAK, which may be redefined. By default, it is simply a "break", to separate each rule's action from the following rule's.  Redefining YY_BREAK allows, for example, C++ users to #define YY_BREAK to do nothing (while being very careful that every rule ends with a "break" or a "return"!) to avoid suffering from unreachable statement warnings where because a rule's action ends with "return", the YY_BREAK is inaccessible.
This section summarizes the various values available to the user in the rule actions.
-      char *yytext holds the text of the current token.  It may be modified but not lengthened (you cannot append characters to the end).
If the special directive %array appears in the first section of the scanner description, then yytext is instead declared char yytext[YYLMAX], where YYLMAX is a macro definition that you can redefine in the first section if you don't like the default value (generally 8KB).  Using %array results in somewhat slower scanners, but the value of yytext becomes immune to calls to input() and unput(), which potentially destroy its value when yytext is a character pointer.  The opposite of %array is %pointer, which is the default.
You cannot use %array when generating C++ scanner classes (the -+ flag).
-      int yyleng holds the length of the current token.
-      FILE *yyin is the file which by default flex reads from.  It may be redefined but doing so only makes sense before scanning begins or after an EOF has been encountered.  Changing it in the midst of scanning will have unexpected results since flex buffers its input; use yyrestart() instead.  Once scanning terminates because an end-of-file has been seen, you can assign yyin at the new input file and then call the scanner again to continue scanning.
-      void yyrestart( FILE *new_file ) may be called to point yyin at the new input file.  The switch-over to the new file is immediate (any previously buffered-up input is lost).  Note that calling yyrestart() with yyin as an argument thus throws away the current input buffer and continues scanning the same input file.
-      FILE *yyout is the file to which ECHO actions are done.  It can be reassigned by the user.
-      YY_CURRENT_BUFFER returns a YY_BUFFER_STATE handle to the current buffer.
-      YY_START returns an integer value corresponding to the current start condition.  You can subsequently use this value with BEGIN to return to that start condition.
One of the main uses of flex is as a companion to the yacc parser- generator.  yacc parsers expect to call a routine named yylex() to find the next input token.  The routine is supposed to return the type of the next token as well as putting any associated value in the global yylval. To use flex with yacc, one specifies the -d option to yacc to instruct it to generate the file y.tab.h containing definitions of all the %tokens appearing in the yacc input.  This file is then included in the flex scanner.  For example, if one of the tokens is "TOK_NUMBER", part of the scanner might look like:
%{ #include "y.tab.h" %}
[0-9]+        yylval = atoi( yytext ); return TOK_NUMBER;
flex has the following options:
-b     Generate backing-up information to lex.backup.  This is a list of scanner states which require backing up and the input characters on which they do so.  By adding rules one can remove backing-up states.  If all backing-up states are eliminated and -Cf or -CF is used, the generated scanner will run faster (see the -p flag). Only users who wish to squeeze every last cycle out of their scanners need worry about this option.  (See the section on Performance Considerations below.)
-c     is a do-nothing, deprecated option included for POSIX compliance.
-d     makes the generated scanner run in debug mode.  Whenever a pattern is recognized and the global yy_flex_debug is non-zero (which is the default), the scanner will write to stderr a line of the form:
--accepting rule at line 53 ("the matched text")
The line number refers to the location of the rule in the file defining the scanner (i.e., the file that was fed to flex). Messages are also generated when the scanner backs up, accepts the default rule, reaches the end of its input buffer (or encounters a NUL; at this point, the two look the same as far as the scanner's concerned), or reaches an end-of-file.
-f     specifies fast scanner.  No table compression is done and stdio is bypassed.  The result is large but fast.  This option is equivalent to -Cfr (see below).
-h     generates a "help" summary of flex's options to stdout and then exits.  -?  and --help are synonyms for -h.
-i     instructs flex to generate a case-insensitive scanner.  The case of letters given in the flex input patterns will be ignored, and tokens in the input will be matched regardless of case.  The matched text given in yytext will have the preserved case (i.e., it will not be folded).
-l     turns on maximum compatibility with the original AT&T lex implementation.  Note that this does not mean full compatibility. Use of this option costs a considerable amount of performance, and it cannot be used with the -+, -f, -F, -Cf, or -CF options.  For details on the compatibilities it provides, see the section "Incompatibilities With Lex And POSIX" below.  This option also results in the name YY_FLEX_LEX_COMPAT being #define'd in the generated scanner.
-n     is another do-nothing, deprecated option included only for POSIX compliance.
-p     generates a performance report to stderr.  The report consists of comments regarding features of the flex input file which will cause a serious loss of performance in the resulting scanner.  If you give the flag twice, you will also get comments regarding features that lead to minor performance losses.
Note that the use of REJECT, %option yylineno, and variable trailing context (see the Deficiencies / Bugs section below) entails a substantial performance penalty; use of yymore(), the ^ operator, and the -I flag entail minor performance penalties.
-s     causes the default rule (that unmatched scanner input is echoed to stdout) to be suppressed.  If the scanner encounters input that does not match any of its rules, it aborts with an error.  This option is useful for finding holes in a scanner's rule set.
-t     instructs flex to write the scanner it generates to standard output instead of lex.yy.c.
-v     specifies that flex should write to stderr a summary of statistics regarding the scanner it generates.  Most of the statistics are meaningless to the casual flex user, but the first line identifies the version of flex (same as reported by -V), and the next line the flags used when generating the scanner, including those that are on by default.
-w     suppresses warning messages.
-B     instructs flex to generate a batch scanner, the opposite of interactive scanners generated by -I (see below).  In general, you use -B when you are certain that your scanner will never be used interactively, and you want to squeeze a little more performance out of it.  If your goal is instead to squeeze out a lot more performance, you should  be using the -Cf or -CF options (discussed below), which turn on -B automatically anyway.
-F     specifies that the fast scanner table representation should be used (and stdio bypassed).  This representation is about as fast as the full table representation (-f), and for some sets of patterns will be considerably smaller (and for others, larger). In general, if the pattern set contains both "keywords" and a catch-all, "identifier" rule, such as in the set:
"case"    return TOK_CASE; "switch"  return TOK_SWITCH; ... "default" return TOK_DEFAULT; [a-z]+    return TOK_ID;
then you're better off using the full table representation.  If only the "identifier" rule is present and you then use a hash table or some such to detect the keywords, you're better off using -F.
This option is equivalent to -CFr (see below).  It cannot be used with -+.
-I     instructs flex to generate an interactive scanner.  An interactive scanner is one that only looks ahead to decide what token has been matched if it absolutely must.  It turns out that always looking one extra character ahead, even if the scanner has already seen enough text to disambiguate the current token, is a bit faster than only looking ahead when necessary.  But scanners that always look ahead give dreadful interactive performance; for example, when a user types a newline, it is not recognized as a newline token until they enter another token, which often means typing in another whole line.
Flex scanners default to interactive unless you use the -Cf or -CF table-compression options (see below).  That's because if you're looking for high-performance you should be using one of these options, so if you didn't, flex assumes you'd rather trade off a bit of run-time performance for intuitive interactive behavior. Note also that you cannot use -I in conjunction with -Cf or -CF. Thus, this option is not really needed; it is on by default for all those cases in which it is allowed.
You can force a scanner to not be interactive by using -B (see above).
-L     instructs flex not to generate #line directives.  Without this option, flex peppers the generated scanner with #line directives so error messages in the actions will be correctly located with respect to either the original flex input file (if the errors are due to code in the input file), or lex.yy.c (if the errors are flex's fault -- you should report these sorts of errors to the email address given below).
-T     makes flex run in trace mode.  It will generate a lot of messages to stderr concerning the form of the input and the resultant non- deterministic and deterministic finite automata.  This option is mostly for use in maintaining flex.
-V     prints the version number to stdout and exits.  --version is a synonym for -V.
-7     instructs flex to generate a 7-bit scanner, i.e., one which can only recognized 7-bit characters in its input.  The advantage of using -7 is that the scanner's tables can be up to half the size of those generated using the -8 option (see below).  The disadvantage is that such scanners often hang or crash if their input contains an 8-bit character.
Note, however, that unless you generate your scanner using the -Cf or -CF table compression options, use of -7 will save only a small amount of table space, and make your scanner considerably less portable.  Flex's default behavior is to generate an 8-bit scanner unless you use the -Cf or -CF, in which case flex defaults to generating 7-bit scanners unless your site was always configured to generate 8-bit scanners (as will often be the case with non-USA sites).  You can tell whether flex generated a 7-bit or an 8-bit scanner by inspecting the flag summary in the -v output as described above.
Note that if you use -Cfe or -CFe (those table compression options, but also using equivalence classes as discussed see below), flex still defaults to generating an 8-bit scanner, since usually with these compression options full 8-bit tables are not much more expensive than 7-bit tables.
-8     instructs flex to generate an 8-bit scanner, i.e., one which can recognize 8-bit characters.  This flag is only needed for scanners generated using -Cf or -CF, as otherwise flex defaults to generating an 8-bit scanner anyway.
See the discussion of -7 above for flex's default behavior and the tradeoffs between 7-bit and 8-bit scanners.
-+     specifies that you want flex to generate a C++ scanner class.  See the section on Generating C++ Scanners below for details.
-C[aefFmr] controls the degree of table compression and, more generally, trade-offs between small scanners and fast scanners.
-Ca ("align") instructs flex to trade off larger tables in the generated scanner for faster performance because the elements of the tables are better aligned for memory access and computation. On some RISC architectures, fetching and manipulating longwords is more efficient than with smaller-sized units such as shortwords. This option can double the size of the tables used by your scanner.
-Ce directs flex to construct equivalence classes, i.e., sets of characters which have identical lexical properties (for example, if the only appearance of digits in the flex input is in the character class "[0-9]" then the digits '0', '1', ..., '9' will all be put in the same equivalence class).  Equivalence classes usually give dramatic reductions in the final table/object file sizes (typically a factor of 2-5) and are pretty cheap performance-wise (one array look-up per character scanned).
-Cf specifies that the full scanner tables should be generated - flex should not compress the tables by taking advantages of similar transition functions for different states.
-CF specifies that the alternate fast scanner representation (described above under the -F flag) should be used.  This option cannot be used with -+.
-Cm directs flex to construct meta-equivalence classes, which are sets of equivalence classes (or characters, if equivalence classes are not being used) that are commonly used together.  Meta- equivalence classes are often a big win when using compressed tables, but they have a moderate performance impact (one or two "if" tests and one array look-up per character scanned).
-Cr causes the generated scanner to bypass use of the standard I/O library (stdio) for input.  Instead of calling fread() or getc(), the scanner will use the read() system call, resulting in a performance gain which varies from system to system, but in general is probably negligible unless you are also using -Cf or -CF.  Using -Cr can cause strange behavior if, for example, you read from yyin using stdio prior to calling the scanner (because the scanner will miss whatever text your previous reads left in the stdio input buffer).
-Cr has no effect if you define YY_INPUT (see The Generated Scanner above).
A lone -C specifies that the scanner tables should be compressed but neither equivalence classes nor meta-equivalence classes should be used.
The options -Cf or -CF and -Cm do not make sense together - there is no opportunity for meta-equivalence classes if the table is not being compressed.  Otherwise the options may be freely mixed, and are cumulative.
The default setting is -Cem, which specifies that flex should generate equivalence classes and meta-equivalence classes.  This setting provides the highest degree of table compression.  You can trade off faster-executing scanners at the cost of larger tables with the following generally being true:
slowest & smallest -Cem -Cm -Ce -C -C{f,F}e -C{f,F} -C{f,F}a fastest & largest
Note that scanners with the smallest tables are usually generated and compiled the quickest, so during development you will usually want to use the default, maximal compression.
-Cfe is often a good compromise between speed and size for production scanners.
-ooutput directs flex to write the scanner to the file output instead of lex.yy.c.  If you combine -o with the -t option, then the scanner is written to stdout but its #line directives (see the -L option above) refer to the file output.
-Pprefix changes the default yy prefix used by flex for all globally- visible variable and function names to instead be prefix.  For example, -Pfoo changes the name of yytext to footext.  It also changes the name of the default output file from lex.yy.c to lex.foo.c.  Here are all of the names affected:
yy_create_buffer yy_delete_buffer yy_flex_debug yy_init_buffer yy_flush_buffer yy_load_buffer_state yy_switch_to_buffer yyin yyleng yylex yylineno yyout yyrestart yytext yywrap
(If you are using a C++ scanner, then only yywrap and yyFlexLexer are affected.)  Within your scanner itself, you can still refer to the global variables and functions using either version of their name; but externally, they have the modified name.
This option lets you easily link together multiple flex programs into the same executable.  Note, though, that using this option also renames yywrap(), so you now must either provide your own (appropriately-named) version of the routine for your scanner, or use %option noyywrap, as linking with -lfl no longer provides one for you by default.
-Sskeleton_file overrides the default skeleton file from which flex constructs its scanners.  You'll never need this option unless you are doing flex maintenance or development.
flex also provides a mechanism for controlling options within the scanner specification itself, rather than from the flex command-line.  This is done by including %option directives in the first section of the scanner specification.  You can specify multiple options with a single %option directive, and multiple directives in the first section of your flex input file.
Most options are given simply as names, optionally preceded by the word "no" (with no intervening whitespace) to negate their meaning.  A number are equivalent to flex flags or their negation:
7bit            -7 option 8bit            -8 option align           -Ca option backup          -b option batch           -B option c++             -+ option
caseful or case-sensitive  opposite of -i (default)
case-insensitive or caseless        -i option
debug           -d option default         opposite of -s option ecs             -Ce option fast            -F option full            -f option interactive     -I option lex-compat      -l option meta-ecs        -Cm option perf-report     -p option read            -Cr option stdout          -t option verbose         -v option warn            opposite of -w option (use "%option nowarn" for -w)
array           equivalent to "%array" pointer         equivalent to "%pointer" (default)
Some %option's provide features otherwise not available:
always-interactive instructs flex to generate a scanner which always considers its input "interactive".  Normally, on each new input file the scanner calls isatty() in an attempt to determine whether the scanner's input source is interactive and thus should be read a character at a time.  When this option is used, however, then no such call is made.
main   directs flex to provide a default main() program for the scanner, which simply calls yylex().  This option implies noyywrap (see below).
never-interactive instructs flex to generate a scanner which never considers its input "interactive" (again, no call made to isatty()).  This is the opposite of always-interactive.
stack  enables the use of start condition stacks (see Start Conditions above).
stdinit if set (i.e., %option stdinit) initializes yyin and yyout to stdin and stdout, instead of the default of nil.  Some existing lex programs depend on this behavior, even though it is not compliant with ANSI C, which does not require stdin and stdout to be compile-time constant. In a reentrant scanner, however, this is not a problem since initialization     is performed in yylex_init at runtime.
yylineno directs flex to generate a scanner that maintains the number of the current line read from its input in the global variable yylineno.  This option is implied by %option lex-compat.
yywrap if unset (i.e., %option noyywrap), makes the scanner not call yywrap() upon an end-of-file, but simply assume that there are no more files to scan (until the user points yyin at a new file and calls yylex() again).
flex scans your rule actions to determine whether you use the REJECT or yymore() features.  The reject and yymore options are available to override its decision as to whether you use the options, either by setting them (e.g., %option reject) to indicate the feature is indeed used, or unsetting them to indicate it actually is not used (e.g., %option noyymore).
Three options take string-delimited values, offset with '=':
%option outfile="ABC"
is equivalent to -oABC, and
%option prefix="XYZ"
is equivalent to -PXYZ.  Finally,
%option yyclass="foo"
only applies when generating a C++ scanner ( -+ option).  It informs flex that you have derived foo as a subclass of yyFlexLexer, so flex will place your actions in the member function foo::yylex() instead of yyFlexLexer::yylex().  It also generates a yyFlexLexer::yylex() member function that emits a run-time error (by invoking yyFlexLexer::LexerError()) if called.  See Generating C++ Scanners, below, for additional information.
A number of options are available for lint purists who want to suppress the appearance of unneeded routines in the generated scanner.  Each of the following, if unset (e.g., %option nounput ), results in the corresponding routine not appearing in the generated scanner:
input, unput yy_push_state, yy_pop_state, yy_top_state yy_scan_buffer, yy_scan_bytes, yy_scan_string
(though yy_push_state() and friends won't appear anyway unless you use %option stack).
The main design goal of flex is that it generate high-performance scanners.  It has been optimized for dealing well with large sets of rules.  Aside from the effects on scanner speed of the table compression -C options outlined above, there are a number of options/actions which degrade performance.  These are, from most expensive to least:
REJECT %option yylineno arbitrary trailing context
pattern sets that require backing up %array %option interactive %option always-interactive
'^' beginning-of-line operator yymore()
with the first three all being quite expensive and the last two being quite cheap.  Note also that unput() is implemented as a routine call that potentially does quite a bit of work, while yyless() is a quite- cheap macro; so if just putting back some excess text you scanned, use yyless().
REJECT should be avoided at all costs when performance is important.  It is a particularly expensive option.
Getting rid of backing up is messy and often may be an enormous amount of work for a complicated scanner.  In principal, one begins by using the -b flag to generate a lex.backup file.  For example, on the input
%% foo        return TOK_KEYWORD; foobar     return TOK_KEYWORD;
the file looks like:
State #6 is non-accepting - associated rule line numbers: 2       3 out-transitions: [ o ] jam-transitions: EOF [ \001-n  p-\177 ]
State #8 is non-accepting - associated rule line numbers: 3 out-transitions: [ a ] jam-transitions: EOF [ \001-`  b-\177 ]
State #9 is non-accepting - associated rule line numbers: 3 out-transitions: [ r ] jam-transitions: EOF [ \001-q  s-\177 ]
Compressed tables always back up.
The first few lines tell us that there's a scanner state in which it can make a transition on an 'o' but not on any other character, and that in that state the currently scanned text does not match any rule.  The state occurs when trying to match the rules found at lines 2 and 3 in the input file.  If the scanner is in that state and then reads something other than an 'o', it will have to back up to find a rule which is matched. With a bit of headscratching one can see that this must be the state it's in when it has seen "fo".  When this has happened, if anything other than another 'o' is seen, the scanner will have to back up to simply match the 'f' (by the default rule).
The comment regarding State #8 indicates there's a problem when "foob" has been scanned.  Indeed, on any character other than an 'a', the scanner will have to back up to accept "foo".  Similarly, the comment for State #9 concerns when "fooba" has been scanned and an 'r' does not follow.
The final comment reminds us that there's no point going to all the trouble of removing backing up from the rules unless we're using -Cf or -CF, since there's no performance gain doing so with compressed scanners.
The way to remove the backing up is to add "error" rules:
%% foo         return TOK_KEYWORD; foobar      return TOK_KEYWORD;
fooba       | foob        | fo          { /* false alarm, not really a keyword */ return TOK_ID; }
Eliminating backing up among a list of keywords can also be done using a "catch-all" rule:
[a-z]+      return TOK_ID;
This is usually the best solution when appropriate.
Backing up messages tend to cascade.  With a complicated set of rules it's not uncommon to get hundreds of messages.  If one can decipher them, though, it often only takes a dozen or so rules to eliminate the backing up (though it's easy to make a mistake and have an error rule accidentally match a valid token.  A possible future flex feature will be to automatically add rules to eliminate backing up).
It's important to keep in mind that you gain the benefits of eliminating backing up only if you eliminate every instance of backing up.  Leaving just one means you gain nothing.
Variable trailing context (where both the leading and trailing parts do not have a fixed length) entails almost the same performance loss as REJECT (i.e., substantial).  So when possible a rule like:
%% mouse|rat/(cat|dog)   run();
is better written:
%% mouse/cat|dog         run(); rat/cat|dog           run();
%% mouse|rat/cat         run(); mouse|rat/dog         run();
Note that here the special '|' action does not provide any savings, and can even make things worse (see Deficiencies / Bugs below).
Another area where the user can increase a scanner's performance (and one that's easier to implement) arises from the fact that the longer the tokens matched, the faster the scanner will run.  This is because with long tokens the processing of most input characters takes place in the (short) inner scanning loop, and does not often have to go through the additional work of setting up the scanning environment (e.g., yytext) for the action.  Recall the scanner for C comments:
<comment>[^*\n]* <comment>"*"+[^*/\n]* <comment>\n             ++line_num; <comment>"*"+"/"        BEGIN(INITIAL);
This could be sped up by writing it as:
<comment>[^*\n]* <comment>[^*\n]*\n      ++line_num; <comment>"*"+[^*/\n]* <comment>"*"+[^*/\n]*\n ++line_num; <comment>"*"+"/"        BEGIN(INITIAL);
Now instead of each newline requiring the processing of another action, recognizing the newlines is "distributed" over the other rules to keep the matched text as long as possible.  Note that adding rules does not slow down the scanner!  The speed of the scanner is independent of the number of rules or (modulo the considerations given at the beginning of this section) how complicated the rules are with regard to operators such as '*' and '|'.
A final example in speeding up a scanner: suppose you want to scan through a file containing identifiers and keywords, one per line and with no other extraneous characters, and recognize all the keywords.  A natural first approach is:
%% asm      | auto     | break    | ... etc ... volatile | while    /* it's a keyword */
.|\n     /* it's not a keyword */
To eliminate the back-tracking, introduce a catch-all rule:
[a-z]+   | .|\n     /* it's not a keyword */
Now, if it's guaranteed that there's exactly one word per line, then we can reduce the total number of matches by a half by merging in the recognition of newlines with that of the other tokens:
%% asm\n    | auto\n   | break\n  | ... etc ... volatile\n | while\n  /* it's a keyword */
[a-z]+\n | .|\n     /* it's not a keyword */
One has to be careful here, as we have now reintroduced backing up into the scanner.  In particular, while we know that there will never be any characters in the input stream other than letters or newlines, flex can't figure this out, and it will plan for possibly needing to back up when it has scanned a token like "auto" and then the next character is something other than a newline or a letter.  Previously it would then just match the "auto" rule and be done, but now it has no "auto" rule, only a "auto\n" rule.  To eliminate the possibility of backing up, we could either duplicate all rules but without final newlines, or, since we never expect to encounter such an input and therefore don't how it's classified, we can introduce one more catch-all rule, this one which doesn't include a newline:
[a-z]+\n | [a-z]+   | .|\n     /* it's not a keyword */
Compiled with -Cf, this is about as fast as one can get a flex scanner to go for this particular problem.
A final note: flex is slow when matching NUL's, particularly when a token contains multiple NUL's.  It's best to write rules which match short amounts of text if it's anticipated that the text will often include NUL's.
Another final note regarding performance: as mentioned above in the section How the Input is Matched, dynamically resizing yytext to accommodate huge tokens is a slow process because it presently requires that the (huge) token be rescanned from the beginning.  Thus if performance is vital, you should attempt to match "large" quantities of text but not "huge" quantities, where the cutoff between the two is at about 8K characters/token.