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

This shouldn't match anything:

re.compile('$^')

So if you replace regexp1, regexp2 and regexp3 with '$^' it will be impossible to find a match. Unless you are using the multi line mode.

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After some tests I found a better solution

re.compile('a^')

It is impossible to match and will fail earlier than the previous solution. You can replace a with any other character and it will always be impossible to match