Regular expression worries

[CSUIDL PROGRAMMEr]

folks
I am new to python, so excuse me if i am asking stupid questions.

I have a txt file  and here are some lines of it

Document<Keyword<date:2006-08-19> Keyword<time:11:00:43>
Keyword<username:YOURBOTNICK> Keyword<data:localhost.localdomain>
Keyword<logon:localhost.localdomain
  > Keyword<date:2006-08-19> Keyword<time:11:00:44> Keyword<sender:>
Keyword<receiver:> Keyword<data::+iwx> Keyword<mode::+iwx

I am writing a python program to replace the tags and word  Document
with Doc.

Here is my python program

#! /usr/local/bin/python

import sys
import string
import re

def replace():
  filename='/root/Desktop/project/chatlog_20060819_110043.xml.txt'
  try:
    fh=open(filename,'r')
  except:
    print 'file not opened'
    sys.exit(1)
  for  l in
open('/root/Desktop/project/chatlog_20060819_110043.xml.txt'):

      l=l.replace("Document", "DOC")
      fh.close()

if __name__=="__main__":
  replace()

But it does not replace Document with Doc in  the txt file

Is there anything wrong i am doing

thanks