[Tutor] Example of use of (?P<name>) and (?P=name) in Python regular expressions?

Michael Fourman michael.fourman at ed.ac.uk
Wed Jan 6 12:40:39 CET 2010

I've used (?P=name) recently in an implementation of the porter2 stemming
algorithm from http://snowball.tartarus.org/algorithms/english/stemmer.html

This includes the rule:
## if the word ends with a double remove the last letter (so hopp -> hop)
where we
## Define a double as one of
## bb   dd   ff   gg   mm   nn   pp   rr   tt

re.sub("^(?P<stem>.*?[aeiouy].*?(?P<dd>[bdfgmnprt]?))(?P=dd)$", "\g<stem>",

implements this rule.



Michael Hannon-2 wrote:
> Greetings.  While looking into the use of regular expressions in Python, I
> saw that it's possible to name match groups using:
>     (?P<name>...)
> and then refer to them using:
>     (?P=name)
> I was able to get this to work in the following, nonsensical, example:
>     >>> x = 'Free Fri Fro From'
>     >>> y = re.sub(r'(?P<test>\bFro\b)', r'Frodo (--matched from
> \g<test>)', x)
>     >>> y
>     'Free Fri Frodo (--matched from Fro) From'
>     >>> 
> But, as you can see, to refer to the match I used the "\g" notation (that
> I found some place on the web).
> I wasn't able to find a way to use the "P=" syntax, and I wasn't able to
> find any working examples of this syntax on the web.
> If you have a working example of the use of the "P=" syntax, will you
> please send it to me?
> Thanks.
> -- Mike
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