Grouping in Regular Expression

Question

I am totally confused by grouping in python. For example:

import re

m = re.search('From:\s+(.+)\s+To:\s+(.*)\s*', 'From: Toronto To: Beijing')
print m.group(0)
print m.group(1)
print m.group(2)

I can get 3 results after I run this program, but I don't know why I get such results:

From: Toronto To: Beijing
Toronto
Beijing

In addition, sometimes when I use the group method of a match-object I will get a warning saying "no such group".

So, can anyone explain the usage of grouping in regular expressions to me in plain language?

"0" is the whole string that matches, "1" is the match in-between the first parenthesis and "2" is the match in-between the second parenthesis — Redu
– Redu, Commented Apr 10, 2016 at 16:45

timgeb · Accepted Answer · 2016-04-10 16:26:59Z

1

When you use parenthesis in your regular expression, this indicates a group. You do that two times, and the groups are named starting at 1 and reading from left to right in your regular expression.

Group 0 is a special group for the whole match.

To make a group non-matching, use (?:something). Demo:

>>> s = '12 34 56'
>>> m = re.search('(\d+)\s+(?:\d+)\s+(\d+)', s)
>>> m.group(0) # everything
'12 34 56'
>>> m.group(1) # content of first matching group
'12'
>>> m.group(2) # content of second matching group
'56'

m.groups() will give you the content of all matching groups, in order:

>>> m.groups()
('12', '56')

answered Apr 10, 2016 at 16:26

timgeb

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RegexPro · Accepted Answer · 2016-04-10 16:27:41Z

1

As you are probably aware, the use of parenthesis in a regular expression creates a capturing group (unless you tell it not to do so, in which case it will be a non-capturing group). So in the regular expression you present you have the 0th group, which is always the full string it captures, the 1st group, which corresponds to the (.+) and the 2nd which corresponds to the (.*)

answered Apr 10, 2016 at 16:27

RegexPro

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