Why is regex so slow?

[Dave Angel]

On 06/18/2013 09:51 PM, Steven D'Aprano wrote:

    <SNIP>
>
> Even if the regex engine is just as efficient at doing simple character
> matching as `in`, and it probably isn't, your regex tries to match all
> eleven characters of "ENQUEUEING" while the `in` test only has to match
> three, "ENQ".
>

The rest of your post was valid, and useful, but there's a misconception 
in this paragraph;  I hope you don't mind me pointing it out.

In general, for simple substring searches, you can search for a large 
string faster than you can search for a smaller one.  I'd expect

if "ENQUEUING" in bigbuffer

to be faster than

if "ENQ"  in bigbuffer

assuming that all occurrences of ENQ will actually match the whole 
thing.  If CPython's implementation doesn't show the speed difference, 
maybe there's some room for optimization.

See Boyer-Moore if you want a peek at the algorithm.

When I was writiing a simple search program, I could typically search 
for a 4-character string faster than REP SCASB could match a one 
character string.  And that's a single instruction (with prefix).
-- 
DaveA