Regular expression syntax for “match nothing”?

Question

I have a python template engine that heavily uses regexp. It uses concatenation like:

re.compile( regexp1 + \"|\" + regexp2 + \"*|\" + regexp3 + \"+\" )
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Answer 1

(?!) should always fail to match. It is the zero-width negative look-ahead. If what is in the parentheses matches then the whole match fails. Given that it has nothing in it, it will fail the match for anything (including nothing).