Question

For example, I have this string: "http://www.google.com/this_is_our_network/"

I want to match against the word “work” but with no alphabetic character in the start or in the end. In the above example the regex should not return a match.

But, in this string: "http://www.google.com/work_for_us.html" the regex should come up with a match since there is no alphabetic character in the start nor in the end.

Answer 1

Try this regex: (?<=[\W_])work(?=[\W_])

This uses positive look-ahead and look-behind assertions to respect enclosing characters but without including them in the match.

This regex matches work

1. if it follows a \W character or an underscore
   
   AND
2. if it is followed by a \W character or an underscore.

\b for word boundary matching can’t be used since _ matches \w which is not wanted here.

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Further examples:

• Matching multiple words:
  (?<=[\W_])(work|job)(?=[\W_])

• Same as above but without creating submatches:
  (?<=[\W_])(?:work|job)(?=[\W_])

• Also respecting line end:
  (?<=[\W_])(?:work|job)(?=[\W_]|$)

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Some useful notes regarding regex syntax:

• \w matches all alphanumeric characters and underscore; this is equivalent to [a-zA-Z0-9_]

• \W matches the exact opposite of \w

• \b matches boundaries between a \w and a \W character (or vise-versa)

• Positive look-ahead assertion:
  foo(?=bar) matches foo followed by bar, without including bar in the match.

• Positive look-behind assertion:
  (?<=foo)bar matches bar if it follows foo, without including foo in the match.

For further information on (python) regex syntax consider the python regex docs or the perl regex docs. Also, the web-based Python Regex Tool is handy for testing.