Oh Yes You Can Use Regexes to Parse HTML

Re:Oh Yes You Can Use Regexes to Parse HTML by Terretta on Friday July 08, 2011 @07:53AM (#2742057)

> Now I understood the reason _why_ you can't use regular expressions to parse HTML is that HTML is usually not regular. Is this true?

From the comments:

Q. "The answer is you can't. HTML is not regular so be definition it can't be described by a regular expression."

A. "Your use of REGULAR in regular expressions has been irrelevant and wrong since Ken Thompson first put backrefs into regexes around 40 years ago. /(.)\1/ parses non-REGULAR languages perfectly well. Please stop repeating this nonsense. – tchrist"

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Re:Oh Yes You Can Use Regexes to Parse HTML by adobriyan on Friday July 08, 2011 @08:35AM (#2742170)

Backreferences in some sense don't work because real HTML contains misnesteded tags and other bugs, so many of them, even HTML5 spec explictely mentions correction algo.

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Re:Oh Yes You Can Use Regexes to Parse HTML by FaceKicker on Friday July 08, 2011 @11:52AM (#2742936)

Why is the language matched by /(.)\1/ non-regular? Isn't it finite (aa, bb, cc, dd, ..., zz if your alphabet is the lowercase letters a-z)? And all finite languages are trivially regular.

Maybe he meant /(.*)\1/?

Note: I know that backreferences make Perl regexes capable of matching non-regular languages, but this doesn't seem like an example of that.

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