The YARD parser is a very simple regular expression pattern matcher which can match regular expression in arbitrary data. The YARD parser is designed as a simpler alternative to external tools like Bison and Flex, and is even simpler to use than the Boost Spirit Library. The YARD parser is designed differently than most RD-parsers in that it uses template meta-functions to describe the grammar productions. The grammar productions are combined using regular expression operators. It isn't as complicated as it might seem. The production rules for an identifier in Heron or C++ is expressed as:

  typedef re_or<MatchLetter, MatchChar<'_'> > MatchIdentFirstChar;
  typedef re_or<MatchIdentFirstChar, MatchNumber> MatchIdentOtherChar;
  typedef re_and<MatchIdentFirstChar, re_star<MatchIdentOtherChar> > MatchIdent;

  struct Misc : public 
    re_or3<
      Comment, 
      PI, 
      S
    > 
    { };

  struct prolog : public 
    re_and3<
      re_opt<XMLDecl>, 
      re_star<Misc>, 
      re_opt<
        re_and<
          doctypedecl, 
          re_star<Misc> 
        > 
      >
    >
    { };
    
  struct document : public 
    re_and3<
      prolog, 
      element, 
      re_star<Misc> 
    > 
    { };