It is important to distinguish between PEG and packrat.<p>PEG is a model for recognizers, distinct from traditional grammers whose theoretical model is usually based on generating strings rather than recognising them. (Yes this is a bit backwards.) The most distinctive feature of PEGs is the ordered choice operator - traditional context-free grammars and regexes use unordered choice, but ordered choice is a closer model of how recursive descent parsers work in practice. Ordered choice naturally leads to implementations that have less backtracking than common regex matchers, e.g. Lpeg.<p>Packrat parsers are based on a clever non-backtracking PEG matching algorithm. They spend O(n) memory to get O(n) parsing time, so they are reliably fast but not good for long inputs. They are also effective in a pure, lazy setting like Haskell.<p>Lpeg is a PEG parser but not a packrat parser.