import re
from operator import pos, neg
from random import randint

def rolld20(fmt):
    """Perform a dice roll in the d20 format (e.g. 5d8-3)"""
    x = 0
    op = pos
    for part in re.findall(r'(\d*d\d+|\d+|\+|-)', fmt):
        if 'd' in part:
            mult, die_n = part.split('d')
            mult = int(mult or 1)
            die_n = int(die_n)
            x += op(sum(randint(1, die_n) for _ in range(mult)))
        elif part[0].isdigit():
            x += op(int(part))
        op = pos
        if part == '-':
            op = neg
    return x