How to match an underscore using Python's regex? [duplicate]

Question

I'm having trouble matching the underscore character in Python using regular expressions. Just playing around in the shell, I get:

>>> import re
>>> re.match(r'a', 'abc')
<_sre.SRE_Match object at 0xb746a368>
>>> re.match(r'_', 'ab_c')
>>> re.match(r'[_]', 'ab_c')
>>> re.match(r'\_', 'ab_c')

I would have expected at least one of these to return a match object. Am I doing something wrong?

Answer 1

Use re.search instead of re.match if the pattern you are looking for is not at the start of the search string.

re.match(pattern, string, flags=0)

Try to apply the pattern at the start of the string, returning a match object, or None if no match was found.

re.search(pattern, string, flags=0)

Scan through string looking for a match to the pattern, returning a match object, or None if no match was found.

You don't need to escape _ or even use raw string.

>>> re.search('_', 'ab_c')
Out[4]: <_sre.SRE_Match object; span=(2, 3), match='_'>

Answer 2

Try the following:

re.search(r'\_', 'ab_c')

You were indeed right to escape the underscore character! Mind that you can only use match for the beginning of strings, as is also clear from the documentation (https://docs.python.org/2/library/re.html):

If zero or more characters at the beginning of string match the regular expression pattern, return a corresponding MatchObject instance. Return None if the string does not match the pattern; note that this is different from a zero-length match.

You should use search in this case:

Scan through string looking for the first location where the regular expression pattern produces a match, and return a corresponding MatchObject instance. Return None if no position in the string matches the pattern; note that this is different from finding a zero-length match at some point in the string.