incremental regex replace in python

sort.py

import os, sys, re

pattern = re.compile("<ID>\d+<\/ID>", re.I | re.M)

file = 'EoCApp.CT'
if len(sys.argv) > 1:
  file = sys.argv[1]

f = open(file, 'r')
s = f.read()
f.close()

i = -1
def r(l):
  global i
  i += 1
  return '<ID>' + str(i) + '</ID>'
s = pattern.sub(r, s)

l = file.rfind('.')
file2 = file[:l] + '2.' + file[l+1:]
f = open(file2, 'w')
f.write(s)
f.close()
print file2 + " written."