regex replace group with specific value

Question

I use python 2.7

I just try to change a group in a regex with a value:

import re

r = "/foo/bar/(?P<pk>[0-9]+)/"
rc = re.compile(r)
#that i try to do : rc["pk"] = 42 and get the resut
print rc.groupindex
#return {'pk' : 1}

I need to do this because i don't know the regex, but I know that ther is a group in it.

Edit:

I want to have a result like this:

rc["pk"] = 42
#now rc is /foo/bar/42 because (?P<pk>[0-9]+) is replace with 42

Answer 1

I am not a python programmer, but I work with regexes a great deal in a number of other systems. I believe you can use the re.sub function with backreferences to groups like so:

Search Pattern:

'(/foo/bar/)[0-9]+(/)'

Replacement pattern:

'\g<1>42\g<2>'

This would replace

'/foo/bar/17/'

with

'/foo/bar/42/'

This would even work where the folder names are expressions themselves:

'(/\w+/\w+/)\d+(/)'

Python also supports lookaround statements, like this:

'(?<=/foo/bar/)\d+(?=/)'

Then you just replace the match with '42' . (Lookarounds do not "consume" characters, so the text in '((?<=...)' and '(?=...)' would not be replaced.)