begin is an outputmodifer. It works by searching a file (or std in) for a user supplied pattern and then outputs all lines starting from the first line that matches the pattern. A pattern is a simple textstring or a regular expression as defined by Boost xpressive header only library. Check out http://www.boost.org/doc/libs/1_41_0/doc/html/xpressive/user_s_guide.html for more information on how to build your regular expression.
