Regex: how to match overlapping patterns (maybe Python specific)

Question

I have a string that looks like this: "XaXbXcX". I'm looking to match any lowercase letters surrounded by X on either side. I tried this in Python, but I'm not getting what I'm looking for:

import re
str = "XaXbXcX"
pattern = r'X([a-z])X'
matches = re.findall(pattern, str) # gives me ['a', 'c']. What about b?

In that particular case, you could have use : re.split('X', str). — dugres
– dugres, Commented Mar 4, 2011 at 9:49
@dugres: not really, if the string looked like XaXbXcXddXeeeX, the pattern would fail by returning dd and eee (and some empty string at the beginning and end). — Michael Ekoka
– Michael Ekoka, Commented Mar 4, 2011 at 16:22

Jochen Ritzel · Accepted Answer · 2011-03-04 03:52:04Z

7

You can use a lookbehind assertion:

pattern = r'(?<=X)([a-z])X'

answered Mar 4, 2011 at 3:52

Jochen Ritzel

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2 Comments

Michael Lorton Over a year ago

I believe (but could be convinced otherwise) that lookbehind is a Python 2.7 feature -- which is not to criticize this solution; if you're going to go with RegExes, it's the only way.

rmmh Over a year ago

That is incorrect, lookbehind assertions have been available since Python 2.0. docs.python.org/release/2.0/lib/re-syntax.html

Saravanan · Accepted Answer · 2011-03-04 04:42:29Z

0

I do not know python, however this regex works i tested in gskinner too ([^(?:X)+])+.

Hope this helps you

answered Mar 4, 2011 at 4:42

Saravanan

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3 Comments

Saravanan Over a year ago

the following pattern also works.. ([^X])+ give this a try too and let me know if it helps. The full regex is : /([^(?:X)+])+/gi

Michael Ekoka Over a year ago

Your patterns matches all characters that "are not uppercase X", which is quite different from "all lowercase characters surrounded by an X on either side". e.g. if the string passed looks like this XaXbbXcXdXeXfXggX, it would fail, as it would also match characters b and g, even though they don't have X on both sides.

Saravanan Over a year ago

as i select all the characters other than X, i thought that you could useup all the matches. anyways b,g are all surrounded by X. thanks for your comment

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