What algorithm are you using with the "obvious code"? Recursive descent? That definitely works well in many cases, but not all.

Here are some things that I think are difficult about parsing:

1. The algorithm to use depends heavily on the language, and it takes some experience to know which algorithm to use for which language. Some languages are hard to write predictive recursive descent parsers for -- they can require a lot of backtracking, which is exponential in general. C would be a good example -- it is more efficiently parsed with a bottom-up algorithm than top down (recursive descent), because of the lookahead issue. In contrast, languages like Python or Go are designed to be LL(1) -- you just have to look at a single token -- "def" or "func" -- to know what kind of thing you need to parse next.

Recursive descent was also considered too slow for expression parsing, where you have left recursion. If you have 13 levels of precedence, then you require 13 nested function calls to parse an atom. So you have basically 3 choices of: Pratt parsing, Shunting Yard algorithm, or Precedence Climbing (I've implemented #1 and #3 and like #3 the most). So you have to change the algorithm for the sublanguage of expressions, i.e. compose two different algorithms.

(I think these days nobody cares about 13 levels of function calls, unless you are parsing gigabytes of C++ code. Of course, this happens literally all the time, because of header file explosion ...)

2. If you know exactly what language you are going to parse, writing a hand-written parser is straightforward (although laborious). If you don't know the language, then refactoring parsers can be quite difficult. You need good test cases not to break corner cases.

A Lua paper talked about how until Lua 4.0 they used yacc while they were designing the language, and then when the language became more fixed in Lua 5.0, they switched to a hand-written parser. This seems to be very common. I think gcc also switched from yacc to a hand-written parser.

3. It's not straightforward to combine two parsers for two sublanguages and get a parser for the combined language (or get errors if the languages can't be composed). Many real languages are really several different sublanguages combined. C++ has a normal mathematical expression language, and a template expression language (there was that notorious >> vs foo<bar<T>> ambiguity in C++). Unix sh is about 5 different sublanguages mashed together (not counting external utilities). Parsing HTML requires parsing JS and CSS.

http://tratt.net/laurie/blog/entries/parsing_the_solved_prob...

4. Writing secure parsers. This is an issue for languages like JavaScript, where the input is untrusted. If you write a parser in C, I pretty much guarantee I will find a crash or buffer overflow in it (through fuzzing or otherwise). Basically ALL parsers in production have crash bugs uncovered after YEARS of widespread use, e.g. Python, Clang, etc.

See http://lcamtuf.blogspot.com/2015/04/finding-bugs-in-sqlite-e... . Note how mature sqlite is and how extensively it is tested -- you can still shake obscure bugs out quite easily.

5. Writing reusable parsers.

5a. Try writing a parser that allows you to reformat your source code, with whitespace and comments preserved. Most languages have multiple parsers these days -- i.e. the Java parser in your compiler is not the same parser that's used by the IDE.

5b. Try writing a parser with the hooks necessary to supports code completion and auto-correct. There was a good video by Anders Hejlsberg here that mentioned that these type of parsers have to parse MORE THAN the language, so that they can continue to provide the required functionality when the code in the IDE is malformed.

FWIW I recall that Steve Wozniak also disclaimed any background in parsing algorithms, and that he said he invented his own table driven parsing algorithm for BASIC. That is totally plausible, but I also think he would have a hard time writing certain parsers for certain languages, supporting advanced functionality, without some study.

EDIT: #6: The line between the lexer and parser is not clear -- it depends on the language being parsed, and sometimes even on the implementation language.

In theory, you have a pipeline from the lexer to the parser. In practice, the lexer and the parser often need to communicate, resulting in circularity. This communication can be hard to reason about and debug.

Moreoever, you can often solve parsing problems (i.e. resolve ambiguity) in either the lexer or the parser. (I think Go's and JavaScript's semicolon insertion are done in the lexer) If you pick the wrong choice, then you can have bugs or a lot of extra complexity. It takes experience and thought to get these things right.

(Side note: depending on the language, there can be more stages than just lexing and parsing. Unix shell has at least 3 stages.)