Forward slash in a Python regex

Question

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?

Answer 1

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.

Answer 2

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'.

Answer 3

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.

Answer 4

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.