问题描述
阅读乔木斯基层次结构 ......我知道regexp不能解析2型语法(无上下文语法)，也不能解析1型和0型语法.正则表达式可以解析/捕获所有3类语法(常规语法)吗?
Reading Chomsky hierarchy ...... I know regexp can't parse type-2 grammars (context-free grammars), and also type-1 and type-0.Can regular expressions parse/catch ALL type-3 grammars (regular grammars)?
推荐答案
是的，只要它们支持交替，串联和Kleene星. PCRE(Perl/Java/JavaScript/PHP/...)类型的正则表达式就是这种情况:交替由((...)|(...))实现，串联由(...)(...)实现，而Kleene star由(...)*实现. (在大多数这些语言中，还有一些其他细节—您需要使用\A和\z之类的东西来表示字符串开始"和字符串结束"，以常规语法表示被认为是理所当然的—就是这个主意.)
Yes, provided they support alternation, concatenation, and the Kleene star. This is the case for regexes of the PCRE (Perl/Java/JavaScript/PHP/...) type: alternation is implemented by ((...)|(...)), concatenation by (...)(...), and the Kleene star by (...)*. (There are a few other details — in most of these languages you need to use something like \A and \z to indicate "start-of-string" and "end-of-string", which in a regular grammar is taken for granted — but that's the idea.)
但是在编程环境中并不是所有被称为正则表达式"的东西都必须具有上述所有内容；例如， POSIX基本正则表达式仅支持非常有限的交替形式，其中所有交替的分支"由一个字符组成(例如，PCRE同时具有(a|b|c)和特例等效的[abc]，而POSIX BRE仅具有[abc]，因此不能表示类似(ab|c)的内容)
But not everything called a "regular expression" in a programming context necessarily has all of the above; for example, POSIX Basic Regular Expressions supports only a very limited form of alternation, where all "branches" of the alternation consist of a single character (e.g., whereas PCREs has both (a|b|c) and the special-case-equivalent [abc], POSIX BREs only have [abc], so can't express something like (ab|c)).
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