非常简单的解析表达式语法实现

Piraha gets its name from one of the simpler spoken languages. Piraha is intended to supply an api that looks and works very similarly to a regular expression api, but is actually capable of parsing full grammars. Piraha is intended to make writing grammars as easy as writing regular expressions. We call them "Pegular Expressions" (thanks to Larry Cox for inventing this term). See QuickStart for an introduction to matching patterns with Piraha, and Summary for a reference card.