import re
from math import ceil
import hashlib


def base64Regex(byteLength):
    padLength = (3 - (byteLength % 3)) if (byteLength % 3) > 0 else 0
    strLength = int(ceil(byteLength / 3.)) * 4 - padLength
    padding = '=' * padLength
    regexString = '[a-zA-Z0-9+/]{{{strLength}}}{padding}'.format(**locals())
    return re.compile(regexString)


def base64RegexForHash(hashType):
    if hashType not in hashlib.algorithms_available:
        raise RuntimeError('Invalid hash type: {}'.format(hashType))
    byteLength = len(hashlib.new(hashType).digest())
    return base64Regex(byteLength)


# example usage:
sha1_b64_regex = base64RegexForHash('sha1')  # r'[a-zA-Z0-9+/]{27}='