CODEWITHSUNDEEP
pythonregex
Doboy
Doboy

Reputation: 11122

python regular expression

whats the difference between '{m}' and '{m,n}?' in http://docs.python.org/library/re.html it says '{m,n}?' matches numbers in range m to n times, but it is not a greedy search. Therefore if its not a greedy search wouldn't it only match up to m no matter what?

Upvotes: 3

Views: 1771

Answers (2)

Glenn Maynard
Glenn Maynard

Reputation: 57544

It's easiest to see with an example using two matching groups:

>>> re.match(r'(x{1,3}?)(.*)', 'xxxxx').groups()
('x', 'xxxx')
>>> re.match(r'(x{1,3})(.*)', 'xxxxx').groups()
('xxx', 'xx')

In other words, {n,m} and {n,m}? are both able to match exactly the same things; what it changes is where the groupings happen when there's more than one way to match.

Upvotes: 1

Tim Pietzcker
Tim Pietzcker

Reputation: 336478

{m,n}? will preferably match only m repetitions, but it will expand as needed up to n repetitions if that's necessary for a longer match.

Compare ^x{2}y$ and ^x{2,4}?y$:

The former will fail on xxxy whereas the latter will match.

To summarize:

x{m}: Match x exactly m times.

x{m,n}: Try to match x n times, but if that causes the overall match to fail, give back as needed, but match at least m times (greedy quantifier).

x{m,n}?: Try to match x m times, but if that causes the overall match to fail, expand as needed, but match at most n times (lazy quantifier).

Upvotes: 14

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