Question

Perhaps a silly question, but though google returned lots of similar cases, I could not find this exact situation: what regular expression will match all string NOT containing a particular string. For example, I want to match any string that does not contain ‘foo_’.
Now,

 re.match('(?<!foo_).*', 'foo_bar') 

returns a match. While

re.match('(?<!foo_)bar', 'foo_bar')

does not.
I tried the non-greedy version:

 re.match('(?<!foo_).*?', 'foo_bar')

still returns a match.
If I add more characters after the ),

re.search('(?<!foo_)b.*', 'foo_bar')

it returns None, but if the target string has more trailing chars:

re.search('(?<!foo_)b.*', 'foo_barbaric')

it returns a match.
I intentionally kept out the initial .* or .*? in the re. But same thing happens with that.

Any ideas why this strange behaviour? (I need this as a single regular expression – to be entered as a user input).

Answer 1

You’re using lookbehind assertions where you need lookahead assertions:

re.match(r"(?!.*foo_).*", "foo_bar")

would work (i. e. not match).

(?!.*foo_) means “Assert that it is impossible to match .*foo_ from the current position in the string. Since you’re using re.match(), that position is automatically defined as the start of the string.