I need to write a regular expression to get all the characters in the list below.. (remove all the characters not in the list)
allow_characters = "#.-_abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789" I don't know how to do it, should I even use re.match or re.findall or re.sub...?
Thanks a lot in advance.
Don't use regular expressions at all, first convert allow_characters to a set and then use ''.join() with a generator expression that strips out the unwanted characters. Assuming the string you are transforming is called s:
allow_char_set = set(allow_characters) s = ''.join(c for c in s if c in allow_char_set) That being said, here is how this might look with regex:
s = re.sub(r'[^#.\-_a-zA-Z0-9]+', '', s) You could convert your allow_characters string into this regex, but I think the first solution is significantly more straightforward.
Edit: As pointed out by DSM in comments, str.translate() is often a very good way to do something like this. In this case it is slightly complicated but you can still use it like this:
import string allow_characters = "#.-_abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789" all_characters = string.maketrans('', '') delete_characters = all_characters.translate(None, allow_characters) s = s.translate(None, delete_characters) 
allow_characters string into this regex" isn't as complicated as it sounds; it's just r'[^{}]+'.format(allow_characters). But I agree that the first solution is clearer.
[a-z] matches all lowercase letters, and [a\-z] would match "a", "-", or "z". You can use re.escape() to automatically do the escaping, just make sure this is the first step in the transformation (before you add the [ and ]+ to the ends).allow_characters by removing everything else.