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The 'owl' book (mastering regular expressions) is a great text for questions like this, and my answer comes from it (paraphrased):

No, you can't really recognize a valid email address with a regex, because technically an email address can have arbitrarily nested comments in parentheses, and a regular expression can never recognize arbitrarily deep nested structures. When you start talking about balanced constructs, you are out of the land of regular languages and into the land of context free languages.

I wonder if it would be useful or just unnecessary to have native support for context free grammars in perl...

To recognize all valid email address that have less than or equal to 1 level of comments requires something like a 5000 byte regular expression.

The moral of this story is that regex's can't do everything.

-Mark


In reply to RE: Re: Common RegExps by young perlhopper
in thread Common RegExps by Anonymous Monk

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