This is a parsing library and language specifier. It uses packrat parsing, as opposed to LL(k) or LR(k) parsing. Packrat parsing uses memoization in a recursive decent parser. By storing the production results from each significant point it speeds up the parse. PEG is a formalized grammar specification optimized for packrat parsing. Peggy also allows user to specfy their grammar in pure Ruby as methods or using a Builder. And the default Peggy grammar is a varitaion on PEG, with support for full regular expressions and for simplifed grammars which automatically ignore a set of productions.