Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

Global _

boost::xpressive::_ — Matches any one character.

unspecified _;

Match any character, similar to '.' in perl syntax with the /s modifier. '_' matches any one character, including the newline.