CODEWITHSUNDEEP
pythonregex
exupero
exupero

Reputation: 9426

Forward slash in a Python regex

I'm trying to use a Python regex to find a mathematical expression in a string. The problem is that the forward slash seems to do something unexpected. I'd have thought that [\w\d\s+-/*]* would work for finding math expressions, but it finds commas too for some reason. A bit of experimenting reveals that forward slashes are the culprit. For example:

>>> import re
>>> re.sub(r'[/]*', 'a', 'bcd')
'abacada'

Apparently forward slashes match between characters (even when it is in a character class, though only when the asterisk is present). Back slashes do not escape them. I've hunted for a while and not found any documentation on it. Any pointers?

Upvotes: 30

Views: 86582

Answers (4)

Ber
Ber

Reputation: 41873

Look here for documentation on Python's re module.

I think it is not the /, but rather the - in your first character class: [+-/] matches +, / and any ASCII value between, which happen to include the comma.

Maybe this hint from the docs help:

If you want to include a ']' or a '-' inside a set, precede it with a backslash, or place it as the first character.

Upvotes: 36

Stephan202
Stephan202

Reputation: 61609

The * matches its argument zero or more times, and thus matches the empty string. The empty string is (logically) between any two consecutive characters. Hence

>>> import re
>>> re.sub(r'x*', 'a', 'bcd')
'abacada'

As for the forward slash, it receives no special treatment:

>>> re.sub(r'/', 'a', 'b/c/d')
'bacad'

The documentation describes the syntax of regular expressions in Python. As you can see, the forward slash has no special function.

The reason that [\w\d\s+-/*]* also finds comma's, is because inside square brackets the dash - denotes a range. In this case you don't want all characters between + and /, but a the literal characters +, - and /. So write the dash as the last character: [\w\d\s+/*-]*. That should fix it.

Upvotes: 3

Lukáš Lalinský
Lukáš Lalinský

Reputation: 41316

You are saying it to replace zero or more slashes with 'a'. So it does replace each "no character" with 'a'. :)

You probably meant [/]+, i.e. one or more slashes.

EDIT: Read Ber's answer for a solution to the original problem. I didn't read the whole question carefully enough.

Upvotes: 10

jamessan
jamessan

Reputation: 42757

r'[/]*' means "Match 0 or more forward-slashes". There are exactly 0 forward-slashes between 'b' & 'c' and between 'c' & 'd'. Hence, those matches are replaced with 'a'.

Upvotes: 4

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