I know this is probably pretty basic, but I'm trying to create a regex expression that will only match a certain multiple of a group of characters. For example, re.findall(expression, 'aaaa') will return 'aaaa' but re.findall(expression, 'aaa') will return 'aa', where expression is some regex that involves the pair aa. It will only return the entire string if the entire string is some integer multiple of 'aa'. Any ideas?
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For non-capturing groups you might read at here.Igor Savinkin– Igor Savinkin2012-08-05 17:35:09 +00:00Commented Aug 5, 2012 at 17:35
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2 Answers
Just use (aa)+. (For findall, you'll want to use non capturing groups, so (?:aa)+.)
>>> re.findall('(?:aa)+', 'aa')
['aa']
>>> re.findall('(?:aa)+', 'aaaa')
['aaaa']
>>> re.findall('(?:aa)+', 'aaaaa')
['aaaa']
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rottentomato56
Thanks I think that's what I was looking for. I didn't know about capturing and non-capturing groups.
Try something like e.g. (?:(?:expression){3})+ to find all multiples of three of the expression. If the expression is shorter, you could also just write it as often as you want.
If you want to match exact duplications, try something like e.g. (?:(expression)\1{2})+ for multiples of three. Note that this may require backtracking if the expression is non-trivial and thus may be slow.
