正则表达式本身不是正则语言.线索是它们包含括号和方括号，因此必须平衡.

Regular expressions are themselves not a regular language. The clue is that they contain parentheses and square brackets and such that must be balanced.

正则表达式本身可以用无上下文语法来描述，并被解析使用 递归下降解析器.

A regular expression itself can be described by a context-free grammar, and parsed with a recursive-descent parser.