How to replace whitespaces with underscore?

I want to replace whitespace with underscore in a string to create nice URLs. So that for example:

"This should be connected" 

Should become

"This_should_be_connected" 

I am using Python with Django. Can this be solved using regular expressions?

You don't need regular expressions. Python has a built-in string method that does what you need:

mystring.replace(" ", "_")

Replacing spaces is fine, but I might suggest going a little further to handle other URL-hostile characters like question marks, apostrophes, exclamation points, etc.

Also note that the general consensus among SEO experts is that dashes are preferred to underscores in URLs.

import re

def urlify(s):

    # Remove all non-word characters (everything except numbers and letters)
    s = re.sub(r"[^\w\s]", '', s)

    # Replace all runs of whitespace with a single dash
    s = re.sub(r"\s+", '-', s)

    return s

# Prints: I-cant-get-no-satisfaction"
print(urlify("I can't get no satisfaction!"))