When is a issue too complex for a regular expression?

Question

Please don\'t answer the obvious, but what are the limit signs that tell us a problem should not be solved using regular expressions?

For example: Why is a complete emai

Answer 1

Regular expressions are a textual representation of finite-state automata. That is to say, they are limited to only non-recursive matching. This means that you can't have any concept of "scope" or "sub-match" in your regexp. Consider the following problem:

(())()

Are all the open parens matched with a close paren?

Obviously, when we look at this as human beings, we can easily see that the answer is "yes". However, no regular expression will be able to reliably answer this question. In order to do this sort of processing, you will need a full pushdown automaton (like a DFA with a stack). This is most commonly found in the guise of a parser such as those generated by ANTLR or Bison.