Question

Let’s say I have these three lines:

the quick brown fox
the brown fox
the quick brown quick fox

Can regex be used to crop out everything in each line except for the word quick?

The end result would look like this:

quick

quickquick

Answer 1

The specifics depend on the language you are using, but here are a few common approaches for doing this with regex (code samples in Python):

• Find all matches of your target string, and then combine each match into a single string:

  >>> import re
  >>> s = 'the quick brown quick fox'
  >>> ''.join(re.findall('quick', s))
  'quickquick'
  

• Construct a regex to match everything except your target string, and then replace each match with an empty string (this is usually much more difficult than the other alternatives listed):

  >>> re.sub('(?!quick|(?<=q)uick|(?<=qu)ick|(?<=qui)ck|(?<=quic)k).', '', s)
  'quickquick'
  

• Use capturing groups to match everything up until an occurrence of a target string, and then replace with just the target string:

  >>> re.sub('.*?(quick|$)', r'\1', s)
  'quickquick'
  

If your string has multiple lines as in your example, you can either split the strings on line breaks first or adapt the solutions to keep line breaks, for example:

>>> s = '''the quick brown fox
... the brown fox
... the quick brown quick fox'''
>>> print ''.join(re.findall('quick|[\r\n]', s))
quick

quickquick
>>> print re.sub('.*?(quick|$)', r'\1', s, flags=re.MULTILINE)
quick

quickquick