What grammar based parser-generator tools exist for ruby?

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'm comfortable with BNF-style specifications.

I've he开发者_高级运维ard of treetop, but it looks a bit alien and verbose compared to yacc...

Purpose: I want to convert my text markup language to a BNF and generate the parsing code.

---

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

---

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

---

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