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Date: Thu, 10 Jul 2014 19:50:43 +1000
From: Cameron Simpson <cs@zip.com.au>
To: python-list@python.org
Subject: Re: How to write match nth grouped subexpression?
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On 10Jul2014 01:57, rxjwg98@gmail.com <rxjwg98@gmail.com> wrote:
>It says that: match checks for a match only at the beginning of the string.
>Then, it also says that: \1...\9	Matches nth grouped subexpression.
>
>I don't know how to write a script to include grouped subexpression in match?

A grouped subexpression is just a portion of a regexp marked out. They serve 
two main purposes: to refer to a portion of the expression as you intend, and 
to mark a section of the regexp for use by a modifier.

Here's an example:

Suppose you're reading a file and trying to match lines like:

   Hours logged: 12

You might use a regexp like this:

   Hours logged: (\d+)

If you match with the expression above, the "\d+" portion will match one or 
more digits i.e. the "12" in the example line earlier. The "(\d+)" is a grouped 
subexpression, the first one (and only one).

If you write a little test script (untested):

   import re
   TESTLINE = "Hours logged: 12"
   regexp = re.compile( r'Hours logged: (\d+)' )
   m = regexp.match(TESTLINE)
   print "subgroup 1:", m.group(1)

that should print "12".

Cheers,
Cameron Simpson <cs@zip.com.au>

Ignorance is preferable to error; and he is less remote from the truth
who believes nothing, than he who believes what is wrong.
         - Thomas Jefferson