i rewrote the code from https://gist.github.com/ryands/1340998 and added roll dropping functionality

dice-parser.py

import random
import re
import sys
from exceptions import ValueError


def roll(sides):
    """ Roll a (sides) sided die and return the value. 1 <= N <= sides """
    return random.randint(1,sides)

def parse_roll(cmd):
    pattern = re.compile(r"(?P<count>[0-9]*)?d(?P<sides>[0-9]+)(?P<drops>d[0-9]*)?")
    match = re.match(pattern, cmd)

    if not match:
        raise ValueError() # invalid input string

    sides = int(match.group('sides'))
    try:
        count = int(match.group('count'))
    except:
        count = 1
    
    rolls = [ roll(sides) for i in range(count) ]
    dropped = []
    try:
        drops = int(match.group('drops')[1:])
    except:
        drops = 0
    for i in range(drops):
        dropped.append(min(rolls))
        rolls.remove(min(rolls))

    return {
        "count": count,
        "sides": sides,
        "drops": drops,
        "total": sum(rolls),
        "rolls": rolls,
        "dropped": dropped
    }
    
if __name__=="__main__":
    print parse_dice(sys.argv[1])