What grammar based parser-generator tools exist for ruby?

Question

What open source (preferably gem-based) parser-generator options do I have in Ruby?

I\'ve used (flex&bison)|(lex&yacc) from C in the past, and I

Answer 1

Citrus is an option - similar but not identical to Treetop in its grammar.

Answer 2

There's also parslet if you want a PEG-based processor

Answer 3

Have you looked at rex and racc, the gem versions of lex and yacc?