You Cannot Parse HTML with Regular Expressions (stackoverflow.com)

Also, because regular expressions are not the right tool for the job sometimes.

("No one should ever use regular expressions" is hyperbolic and wrong.)

The problem is that regular expressions (in addition not being powerful enough to properly handle HTML) are a bit of a maintenance headache. They look like line noise. Most regular expressions don't change properly with local (did you use \w, [a-zA-Z] or \p{Alpha} and which of these did you really mean). And you can't always be sure the regexp is implemented efficiently (by compiling a finite automata instead of search, see http://swtch.com/~rsc/regexp/regexp1.html ). Often a parser generator (of which a lexer is often the first step, which is as powerful as regular expressions) is a much better solution.