What does the 1 in match.group(1) mean?

Question

I met the codes here:

>>> import urllib, re
>>> prefix = "http://www.pythonchallenge.com/pc/def/linkedlist.php?nothing="
>>> findnothing = re.compile(r"nothing is (d+)").search
>>> nothing = '12345'
>>> while True:
...     text = urllib.urlopen(prefix + nothing).read()
...     print text
...     match = findnothing(text)
...     if match:
...         nothing = match.group(1)
...         print "   going to", nothing
...     else:
...         break

What does the 1 in match.group(1) mean?

Asked By: iljyya

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Source

Answer 1

The 1 is the number of the group — denoted by the parentheses pair — in the the expresssion. Your expression has only one group, but imagine matching "123 456" with r"(d+) (d+)": group(1) would be "123", group(2) would be "456" and group(0) is always the whole match, hence "123 456".

Answered By: Michael

Answer 2

The 1 in match.group(1) represents the first parenthesised subgroup. In your case the first of one.

This is well described in the official documentation for re.MatchObject.group() and best illustrated with an example:

>>> m = re.match(r"(w+) (w+)", "Isaac Newton, physicist")
>>> m.group(0)       # The entire match
'Isaac Newton'
>>> m.group(1)       # The first parenthesized subgroup.
'Isaac'
>>> m.group(2)       # The second parenthesized subgroup.
'Newton'
>>> m.group(1, 2)    # Multiple arguments give us a tuple.
('Isaac', 'Newton')

Answered By: johnsyweb

What does the 1 in match.group(1) mean?

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