How do you use a regex in a list comprehension in Python?

Question

I\'m trying to locate all index positions of a string in a list of words and I want the values returned as a list. I would like to find the string if it is on its own, or if

Answer 1

You don't need to assign the result of match back to x. And your match should be on x rather than list.

Also, you need to use re.search instead of re.match, since your the regex pattern '\W*myString\W*' will not match the first element. That's because test; is not matched by \W*. Actually, you only need to test for immediate following and preceding character, and not the complete string.

So, you can rather use word boundaries around the string:

pattern = r'\b' + re.escape(myString) + r'\b'
indices = [i for i, x in enumerate(myList) if re.search(pattern, x)]

Answer 2

There are a few problems with your code. First, you need to match the expr against the list element (x), not against the whole list (myList). Second, in order to insert a variable in the expression, you have to use + (string concatenation). And finally, use raw literals (r'\W) to properly interpet slashes in the expr:

import re
myList = ['test;cow', 'one', 'two', 'three', 'cow.', 'cow', 'acow']
myString = 'cow'
indices = [i for i, x in enumerate(myList) if re.match(r'\W*' + myString + r'\W*', x)]
print indices

If there are chances that myString contains special regexp characters (like a slash or a dot), you'll also need to apply re.escape to it:

regex = r'\W*' + re.escape(myString) + r'\W*'
indices = [i for i, x in enumerate(myList) if re.match(regex, x)]

As pointed out in the comments, the following might be a better option:

regex = r'\b' + re.escape(myString) + r'\b'
indices = [i for i, x in enumerate(myList) if re.search(regex, x)]