Editing a file by substitution

[Brad]

Hi, what I'd like to do is edit an input file for a calculation with 
Python. Let's say that I have an input file like the following

------------------------------>>

BLAH BLAH BLAH

Other inputs, Volume 940 m^3, maybe some more stuff

STUFF STUFF
------------------------------>>


So let's say I want to edit this file and change Volume from 940 to 950.

This is how I would guess I would do something like it:

---------------------------------------------

import re
expr=r'.*Volume\s+(?P<VolValue>\d+[.]?[\d+]?)'
exprComp=re.compile(expr)
infile=open('myfile','r')
outfile=open('outfile','w')

for line in infile:
      match=exprComp.search(line)
      if match:
          newline=<HELP ME!>
          print >> outfile, newline
      else:
          print >> outfile, line
infile.close()
outfile.close()
------------------------------

So what I would like to do in the section labeled <HELP ME!> is to use 
something like the re.sub() function and replace the contents of the 
"VolValue" group with another number. However I am not sure how exactly 
to do this. I feel like I could use the information that's on this 
website:http://www.regular-expressions.info/python.html to do it but it 
seems that I can only USE captured groups, not REPLACE them. This makes 
it seem to me that I would need my groups to be everything that I WASN'T 
going to replace. Then I could do something like this:

re.sub(regex,\g<StuffBeforeVolumeValue>950\g<StuffAfterVolumeValue>)

or something along those lines. Although that is really so much uglier 
than if i could simply replace the captured group.

Please help pythonistas :)