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

[Martin Walsh]

Michael Hannon 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'm not sure you've got that quite right. IIUC, the (?P=name) syntax is
used to match a previously defined group, "in the regular expression
itself." (http://docs.python.org/library/re.html)

<snip>

x = 'Free Fri Fro Fro From'
y = re.sub(
    r'(?P<test>Fro) (?P=test)',
    r'Frodo (--matched from \g<test>, twice in a row)', x
)
# y == 'Free Fri Frodo (--matched from Fro, twice in a row) From'

> But, as you can see, to refer to the match I used the "\g" notation (that I found some place on the web).

The \g notation is appropriate for re.sub.

HTH,
Marty