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When answering this question (and having read this answer to a similar question), I thought that I knew how Python caches regexes. But then I thought I'd test it, comparing two scenarios: a single compilation of a simple regex, then 10 applications of that compiled regex. 10 applications of an uncompiled regex (where I would have expected slightly worse performance because the regex would have to be compiled once, then cached, and then looked up in the cache 9 times). However, the results were staggering (in Python 3.3): >>> import timeit >>> timeit.timeit(setup="import re", ... stmt='r=re.compile(r"\w+")\nfor i in range(10):\n r.search(" jkdhf ")') 18.547793477671938 >>> timeit.timeit(setup="import re", ... stmt='for i in range(10):\n re.search(r"\w+"," jkdhf ")') 106.47892003890324 That's over 5.7 times slower! In Python 2.7, there is still an increase by a factor of 2.5, which is also more than I would have expected. Has caching of regexes changed between Python 2 and 3? The docs don't seem to suggest that.

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