RecDescent incrementally generates top-down recursive-descent text
parsers from simple yacc-like grammar specifications. It provides:

  * Regular expressions or literal strings as terminals (tokens)
  * Multiple (non-contiguous) productions for any rule
  * Repeated, optional and alternate subrules within productions
  * Late-bound (run-time dispatched) subrules
  * Full access to Perl within actions specified as part of the grammar
  * Simple automated error reporting during generation and parsing
  * The ability to commit to, uncommit to, or reject particular
    productions during a parse
  * Incremental extension of the parsing grammar (even during a parse)
  * The ability to retrieve the generated parsing code.

WWW: http://search.cpan.org/dist/Parse-RecDescent/