Regular expression: match start or whitespace

Can a regular expression match whitespace or the start of a string?

I'm trying to replace currency the abbreviation GBP with a £ symbol. I could just match anything starting GBP, but I'd like to be a bit more conservative, and look for certain delimiters around it.

>>> import re
>>> text = u'GBP 5 Off when you spend GBP75.00'

>>> re.sub(ur'GBP([\W\d])', ur'£\g<1>', text) # matches GBP with any prefix
u'\xa3 5 Off when you spend \xa375.00'

>>> re.sub(ur'^GBP([\W\d])', ur'£\g<1>', text) # matches at start only
u'\xa3 5 Off when you spend GBP75.00'

>>> re.sub(ur'(\W)GBP([\W\d])', ur'\g<1>£\g<2>', text) # matches whitespace prefix only
u'GBP 5 Off when you spend \xa375.00'

Can I do both of the latter examples at the same time?


Solution 1:

Use the OR "|" operator:

>>> re.sub(r'(^|\W)GBP([\W\d])', u'\g<1>£\g<2>', text)
u'\xa3 5 Off when you spend \xa375.00'

Solution 2:

\b is word boundary, which can be a white space, the beginning of a line or a non-alphanumeric symbol (\bGBP\b).

Solution 3:

This replaces GBP if it's preceded by the start of a string or a word boundary (which the start of a string already is), and after GBP comes a numeric value or a word boundary:

re.sub(u'\bGBP(?=\b|\d)', u'£', text)

This removes the need for any unnecessary backreferencing by using a lookahead. Inclusive enough?