A regex to find C/C++ preprocessor includes

Yes I know .. “parsing” C or C++ with regular expressions is bad. There’s of course libclang (1, 2) and also some way to hook into GCC … but sometimes it’s just easier to slap a regex on the problem. In this case getting the names of the directly included files:

grep -oP '#[[:blank:]]*include[[:blank:]]*("|<)\K[\w.\/]+(?=("|>))' FILE

This calls grep with “Perl-compatible” regex support (-P) and prints only the matches (-o). The regex:

• # … literally that character
• [[:blank:]]*: some amount of whitespace; except newlines
• include … literally that word
• [[:blank:]]*: again some whitespace; no newlines
• ("|<): Either " or <
• \K resets the starting point of the reported match. This basically means “only treat the following part of the match as the match” (which is then printed by grep)
• [\w.\/]+ 1 or more of “word characters” (a-z and A-Z), the literal dot . or forward slashes /
• (?= ) is a “positive lookahead” … meaning make sure that this comes next, but don’t add it the the reported match.
• ("|<) (inside the positive lookahead): Either " or >

I’ve found this regex to work well in practice, though of course it may return false positives due to:

• #include in comments; these will be picked up, too
• #include <foo" and #include "foo> will be picked up, too. This could be fixed … but honestly … I don’t think that this is a likely case to happen

And of course it doesn’t care about conditional compilation … so this will yield foo:

#if 0
#include "foo"
#endif

But that’s exactly what I need in my case.