Finding shortest regex match in Python

Question

I want to process a string like this (a (b) c) and I want to match the inner (b) first before processing the outer parens. However, this code doesn't work:

>>> x='(a(b)c)'
>>> re.search(r"\((.*?)\)", x).group(1)
'a(b'

Is there any way to ask Python to find a minimal match (ie, b) rather than the longer match a(b?

Amadan · Accepted Answer · 2016-04-20 22:17:03Z

3

XY problem. You can't process the minimal match. What you can do is find a match without parentheses.

r"\(([^()]*)\)"

answered Apr 20, 2016 at 22:17

Amadan

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7 Comments

Amadan Over a year ago

@WiktorStribiżew: "I want to process the inner (b) first before removing it and processing the outer parens" (emphasis mine). Once you replace the inner expression with a non-parenthesised expression, you can find the outer one. This would typically be done in a loop; no way to do it with one regexp execution.

Fixee Over a year ago

This solves my issue Amadan. Thanks. Side question: what does "minimal" mean in python regex's if not "match as few characters as possible"?

Amadan Over a year ago

Exactly that - but at the place where you're matching, For example, a.+z matched against abcz adz would match abcz adz - The first a available, then as much other characters as possible, then z. Only if the first a couldn't be worked into the match would the second a be tested, like with ad+z. In contrast, a.+?z will match abcz - the first a, as little other characters possible ("minimal"), then z. It will not match adz, because another viable match is available sooner.

Fixee Over a year ago

Perhaps python docs should call it "earliest minimal" or "leftmost minimal" instead of just "minimal" then. I was quite misled by the term "minimal", thinking it meant "smallest possible"

Platinum Fire Over a year ago

Could you not just use "(\(.*?)(\(.*?\))(.*?\))" ? So that group(1) is the middle parameter, and concatenating group(0) and group(2) would get you the entire string without the middle section? Or am I not seeing the problem here?

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