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

Or, use some list comprehension to remove the useless regexp entries and join to put them all together. Something like:

re.compile('|'.join([x for x in [regexp1, regexp2, ...] if x != None]))

Be sure to add some comments next to that line of code though :-)