Replace a regular expression in filename(s) with another expression

subfn

#!/usr/bin/python

import argparse
import os
import re

parser = argparse.ArgumentParser(description='Replace regex in filenames.')
parser.add_argument('files', metavar='F', nargs='+', 
                    help='Files to operate on (Ignores directories)')
parser.add_argument('match', metavar='M', nargs=1, 
                    help='Regular expression to replace in filenames')
parser.add_argument('replace', metavar='R', nargs=1,
                    help='Expression to replace M with.')

args = parser.parse_args()
print args.files
for old in args.files:
    new = re.sub(args.match[0], args.replace[0], old)
    try:
        os.rename(old, new)
    except OSError:
        print "Invalid file"