regex bug, incorrectly matching whitespace characters

[cainbit]

Bug report

Bug description:

>>> import re
>>> pattern = re.compile(r'^\s+([^"].*)')
>>> print(pattern.match('    hello world'))   # case 1
<re.Match object; span=(0, 15), match='    hello world'>
>>> print(pattern.match('    "hello world"')) # case 2
<re.Match object; span=(0, 17), match='    "hello world"'>

Why case 2 is matched?

CPython versions tested on:

3.13

Operating systems tested on:

Linux

[cainbit]

The result looks like '\s+' is not greedy.

[nineteendo]

You're looking for possessive quantifiers: https://regex101.com/r/x69h63/2

>>> import re
>>> pattern = re.compile(r'^\s++([^"].*)')
>>> print(pattern.match('    hello world'))
<re.Match object; span=(0, 15), match='    hello world'>
>>> print(pattern.match('    "hello world"'))
None

• + matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
• ++ matches the previous token between one and unlimited times, as many times as possible, without giving back (possessive)

[picnixz]

Note: in the second case, \s+ matches all but the last whitespace, then [^"] matches the last whitespace, and .* matches "hello world".