Why the python regex pattern fails to match when append a dollar($)?

Question

Here's a small code, using python 3.4

import re
path_pattern=r'(([^\W]|[.~%$])+)'
re.search(path_pattern+'$','./').string

It will report AttributeError: 'NoneType' object has no attribute 'string' on execution.

If I remove the +'$' in the code, It works,

import re
path_pattern=r'(([^\W]|[.~%$])+)'
re.search(path_pattern,'./').string

As far as I know, $ is for matching the end of string, but why isn't it working here?

Your `./` contains `/` that is not matched, thus, there is no match at the end of the string. What are your requirements? (BTW, `[^\W]` = `\w`) — Wiktor Stribiżew, Mar 31 '17 at 06:32
I feel like it matches the actual character in a character set... — juanpa.arrivillaga, Mar 31 '17 at 06:35

score 2 · Answer 1 · answered Mar 31 '17 at 06:45

If your explore your regex path_pattern at https://regex101.com/, you'll find it only matches ., so after you append $, it'll match nothing, and re.search returns None if no position in the string matches the pattern, that's why you get the error.

Check it out here:

>>> path_pattern=r'(([^\W]|[.~%$])+)'
>>> r = re.search(path_pattern + "$",'./')
>>> print(r)
None

score 1 · Accepted Answer · answered Mar 31 '17 at 06:35

1

Your regex cannot match the / character in your string, and only . is matched.

When you use $ in your regex, it cannot match at all. When you remove it, it matches but only with ".".

answered Mar 31 '17 at 06:35

Chuancong Gao

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Why the python regex pattern fails to match when append a dollar($)?

2 Answers2