DnD die roll script

roll.py

#!/usr/bin/env python
if __name__ == '__main__':
  import sys, random, re

  roll_re = r'(?P<rolls>\d+)d(?P<die>\d+)((?P<mod>(\+|-)\d+))?'
  parsed = re.match(roll_re, sys.argv[1].strip()) if len(sys.argv) == 2 else None

  if parsed is not None:
    d = parsed.groupdict()
    rolls, die, mod = int(d['rolls']), int(d['die']), (int(d['mod']) if d['mod'] is not None else 0)

    rng = random.SystemRandom()
    roll = sum(rng.randint(1, die) for _ in range(rolls)) + mod

    print roll
  else:
    print '''roll -- a D'n'D-like dice roller. Handy when you need a random number.

USAGE:
> roll 1d6
3

> roll 1d4+5
6


  How do the D'n'D rolls work?

                  1d4+5
    /-------------^ ^ ^-----------\\
    |               |             |
number of     number of die    modifier
  rolls           faces

  In the example above, we throw a 4-faced die one time,
  then add 5.

  Hint: you can use any number of die faces (unlike in real life).

Enjoy.
'''