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

To match an empty string - even in multiline mode - you can use \A\Z, so:

re.compile('\A\Z|\A\Z*|\A\Z+')

The difference is that \A and \Z are start and end of string, whilst ^ and $ these can match start/end of lines, so $^|$^*|$^+ could potentially match a string containing newlines (if the flag is enabled).

And to fail to match anything (even an empty string), simply attempt to find content before the start of the string, e.g:

re.compile('.\A|.\A*|.\A+')

Since no characters can come before \A (by definition), this will always fail to match.