Question

Given the following string:

"-local locally local test local."

my objective is to replace the string “local” with “we” such that the result becomes

"-local locally we test local."

so far (with the help from the guys here at stackoverflow: Python: find exact match) I have been able to come up with the following regular expression:

variable='local'
re.sub(r'\b%s([\b\s])' %variable, r'we\1', "-local locally local test local.")

However I have two problems with this code:

1. The search goes through the minus sign and the output becomes:

   '-we locally we test local.'
   

   where it should have been

   '-local locally we test local.'
   

2. searching for a string starting with a minus sign such as “-local” fails the search

Answer 1

Try the following:

re.sub(r'(^|\s)%s($|\s)' % re.escape(variable), r'\1we\2', some_string)

The regex that was suggested in the other question is kind of odd, since \b in a character class means a backspace character.

Basically what you have now is a regex that searches for your target string with a word boundary at the beginning (going from a word character to a non-word character or vice versa), and a whitespace character at the end.

Since you don’t want to match the final “local” since it is followed by a period, I don’t think that word boundaries are the way to go here, instead you should look for whitespace or beginning/end of string, which is what the above regex does.

I also used re.escape on the variable, that way if you include a characters in your target string like . or $ that usually have special meanings, they will be escaped and interpreted as literal characters.

Examples:

>>> s = "-local locally local test local."
>>> variable = 'local'
>>> re.sub(r'(^|\s)%s($|\s)' % re.escape(variable), r'\1we\2', s)
'-local locally we test local.'
>>> variable = '-local'
>>> re.sub(r'(^|\s)%s($|\s)' % re.escape(variable), r'\1we\2', s)
'we locally local test local.'