Regex for #else case in C preprocessor directive

Question

I want to find the #else case for certain a C preprocessor define.

Example:

#if defined(my_define)
  // multiple 
  // lines 
  // of 
  // code
#else
  // multiple 
  // lines 
  // of 
  // code
#endif

Or

#if defined (my_define)
  // same as above from here

But I do not want to match a case without #else:

#if defined(my_define)
  // multiple 
  // lines 
  // of 
  // code
#endif

I don't care for nested #ifs, just the cases above.

I tried starting with

defined..?my_define.(\r\n|\r|\n)?

I don't know how to handle the arbitrary number of lines in between the directives.

Thank you. I slightly changed it to #if defined..?my_define\b(?:(?!#(?:end)?if)[\s\S])*#else(?:(?!#(?:end)?if)[\s\S])*#endif to include the defines name (and whitespace) and it works flawless :-) Feel free to write an answer, so I can mark it as solved. — mh333
– mh333, Commented Mar 15, 2017 at 13:06

Wiktor Stribiżew · Accepted Answer · 2017-03-15 13:27:07Z

1

You may use a tempered greedy token solution here:

#if defined..?my_define\b(?:(?!#(?:end)?if)[\s\S])*#else(?:(?!#(?:end)?if)[\s\S])*#endif

Details:

#if defined - a literla char sequence
..? - any 1 or 2 chars other than line break char
my_define\b - a whole word my_define
(?:(?!#(?:end)?if)[\s\S])* - a tempered greedy token matching any char that is not a starting point for an #endif or #if literal char sequence
#else - a literal char sequence
(?:(?!#(?:end)?if)[\s\S])* - same as above
#endif - a literal char sequence

answered Mar 15, 2017 at 13:27

Wiktor Stribiżew

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