\b in Python regular expressions

14 June 2005   3 comments   Python

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Boy did that shut me up! The \b special character i python regular expressions is so useful. I've used it before but have forgotten about it. The following code:

def createStandaloneWordRegex(word):
   """ return a regular expression that can find 'peter'
   only if it's written alone (next to space, start of 
   string, end of string, comma, etc) but not if inside 
   another word like peterbe """
   return re.compile(r"""
     (
     ^ %s
     (?=\W | $)
     |
     (?<=\W)
     %s
     (?=\W | $)
     )
     """% (re.escape(word), re.escape(word)),
           re.I|re.L|re.M|re.X)

can with the \b gadget be simplified to this:

def createStandaloneWordRegex(word):
   """ return a regular expression that can find 'peter'
   only if it's written alone (next to space, start of 
   string, end of string, comma, etc) but not if inside 
   another word like peterbe """
   return re.compile(r'\b%s\b' % word, re.I)

Quite a lot simpler isn't it? The simplified passes all the few unit tests I had.

Comments

Matt Schinckel
Excellent! I was wondering how to do this for a script (http://schinckel.blogsome.com/2005/06/27/ecto-auto-abbracronym/) that automatically adds abbr and acronym tags to text in ecto, a blogging client for MacOS X.
YuppY
First variant could be shorter:

re.compile(r'((?<=\W)|^)%s(?=\W|$)' % re.escape(word), re.I)

re.escape is necessary.
Peter Bengtsson
shorter but certainly less readable.
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