perl-Parse-RecDescent

Parse::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 and optional subrules within productions
 * Full access to Perl within actions specified as part of the grammar
 * Simple automated error reporting during parser generation and parsing
 * The ability to commit to, uncommit to, or reject particular
   productions during a parse
 * The ability to pass data up and down the parse tree ("down" via
   subrule argument lists, "up" via subrule return values)
 * Incremental extension of the parsing grammar (even during a parse)
 * Precompilation of parser objects
 * User-definable reduce-reduce conflict resolution via "scoring" of
   matching productions