counting characters in an object

[Skaperen]

i have a string (str,bytes,bytearray) and an object with one or more characters (str,bytes,bytearray,set,frozenset,list,tuple). the beginning of the string has some number of characters that would get a True result if the in operator is used with that object while stepping through the string until it reaches a character that would get False. as this is hard to explain, here is some code:

def runlen(s,o):
    for n in range(len(s)):
        if s[n] in o:
            continue
        return n

what i would like to know is if there is a way to call something to do that loop internally so it would run faster. maybe re can do this, somehow. once i have the position n, i will be using s[:n] though not s[n:]. so, something that just gives me the prefix would do the job.