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, optional and alternate sub-rules within productions
* late-bound (run-time dispatched) sub-rules
* 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
* incremental extension of the parsing grammar (even during a parse)
* the ability to retrieve the generated parsing code