Match all non-urls using regex look ahead

Question

In Python/django I have a string from which I extract "the title" by matching characters before the ':' character, like:

some_string = "This is my Title: This is some text"

So I'm using this code to extract the title:

result = regex.search('(.*):', some_string)
result.group(1)
>>> 'This is my Title'

There will be problems when a user put only a url in the string, like:

some_string = 'http://vimeo.com/49742318'
result.group(1)
>>> 'http'

I prefer to just have an empty string returened. I've tried using the negative look ahead metatag (?!):

result = regex.search('(.*(?!http)):', some_string)

But it still returns 'http' instead of an empty string. How should it be?

Answer 1

The problem is that at the point where you've put the negative lookahead, the next character is also constrained to be a colon: the negative lookahead succeeds trivially as h is not the next character!

What you probably actually want is to put the negative lookahead after the colon so that the next character is not a /:

(.*):(?!/)

But at that point you might as well use a positive lookahead and stop using a capturing group at all. You should also not allow colons to be captured or the RE would be able to consume much more than you might expect:

result = regex.search('[^:]*(?=:[^/])', some_string)
result.group()