CODEWITHSUNDEEP
pythonregex
russjohnson09
russjohnson09

Reputation: 271

Regular expression beginning and ending with parentheses

Here is a pattern object that I tried to make in python using the re module. What I am going for is something that will take the string "(\exists x)(Px*Qx)" and find just the "Px*Qx" portion. I thought that I would try to use the lookahead and lookbehind assertions. I'm not sure if I am using this wrong or if there is something wrong with the ( character.

p = re.compile(r'?<=[(]\w+?=[)]')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/re.py", line 190, in compile
    return _compile(pattern, flags)
  File "/usr/lib/python2.7/re.py", line 244, in _compile
    raise error, v # invalid expression
sre_constants.error: nothing to repeat

Upvotes: 4

Views: 2384

Answers (1)

mmdemirbas
mmdemirbas

Reputation: 9168

Problems

  1. You need to put tokens into a group to apply lookbehind (add parentheses around): (?<=[(]\w+(?=[)]))

  2. Python doesn't support variable repetition inside lookbehind. So, you can not write \w+ there.

  3. A regex with only a lookbehind matches nothing! Lookbehind means "Try to match this thing, then stay at the same starting position to match following part of the regex." But, in your case, no following part. So, nothing matches.

Solution

If you want to obtain text inside the last parentheses:

^.*\((.*?)\)$

If you want to skip first parentheses and obtain remaining part removing parentheses:

^\(.*?\)\((.*)\)$

Please define what you want more concretely, so I can help you to write a proper regex.

Upvotes: 3

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