How to exclude a character from a regex group?

Question

I want to strip all non-alphanumeric characters EXCEPT the hyphen from a string (python).
How can I change this regular expression to match any non-alphanumeric char except the hyphen?

re.compile('[W_]')

Thanks.

Asked By: atp

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Source

Answer 1

You could just use a negated character class instead:

re.compile(r"[^a-zA-Z0-9-]")

This will match anything that is not in the alphanumeric ranges or a hyphen. It also matches the underscore, as per your current regex.

>>> r = re.compile(r"[^a-zA-Z0-9-]")
>>> s = "some#%te_xt&with--##%--5 hy-phens  *#"
>>> r.sub("",s)
'sometextwith----5hy-phens'

Notice that this also replaces spaces (which may certainly be what you want).

Edit: SilentGhost has suggested it may likely be cheaper for the engine to process with a quantifier, in which case you can simply use:

re.compile(r"[^a-zA-Z0-9-]+")

The + will simply cause any runs of consecutively matched characters to all match (and be replaced) at the same time.

Answered By: eldarerathis

Answer 2

w matches alphanumerics, add in the hyphen, then negate the entire set: r"[^w-]"

Answered By: Ned Batchelder

How to exclude a character from a regex group?

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