Is there a pure regex expression that, given a function name, would capture the body of the function with that name in C/C++?

Question

Let's that we are looking for a function body of foo inside a string or file. Capturing the signature is easy.

However is there a regex expression that would capture the body of a function. We do not make any assumptions about the internal structure of the function.

It could be something flat like:

int foo()
{
    return i; 
}

Or something layered like this:

void foo()
{
    if(true)
    {
       for (int i=0; i<5; i++ )
       {
           cout<< i << std::endl;
       }
    }  
}

So is there a way to do it?

You need a parser to do that. The compiler front-end is a good example of the parser. Regex is not suitable to parse C++ syntax, especially for the function body. — Aliakbar Abbasi, Aug 09 '19 at 09:41
If getting the right answer most of the time is good enough, then just count `{` and `}` taking care to avoid occurances in comments, string and character literals. Not perfect, but may be good enough. — john, Aug 09 '19 at 09:49
@john ...and in preprocessor-disabled sections, and (previously) accounting for trigraphs, and... well, good luck. — Max Langhof, Aug 09 '19 at 09:51

Max Langhof · Answer 1 · 2019-08-09T10:00:40.160

Regex is incapable of doing that (obligatory link).

Simple counterexample: Regex can't parse arbitrarily nested stuff (see link), and

void foo()
{
  struct Helper
  {
    void bar() {}
  };
}

demonstrates that you can have functions nested in functions arbitrarily deep.

And no, matching opening and closing braces (aside from being yet another thing that regex can't do) is insufficient because of comments, strings/character literals and the preprocessor. For example:

#define EVIL {

void foo()
{
  if (someCondition)
  EVIL

    // Closing } is inside the ifdef (and we have another one in this comment).
#ifdef SOME_DEFINE
    doStuff1("I like strings with unmatched braces }");
  } // Who likes trigraphs ??/
  doStuff2();
#else
    doStuff3();
  }
#endif
}

It's totally legal C++. Good luck with a regex.

In general, C++ is very hard to parse and I would recommend using a tool that is built for the job, such as clang's AST matcher.

Is there a pure regex expression that, given a function name, would capture the body of the function with that name in C/C++?

1 Answers1