http://stackoverflow.com/q/10311602/78845

> python --version
Python 2.7.2
> python -m timeit -c '
my_string = "qwer123asd!@#$%^"
predef = set("0x#$qi")

if set(my_string).issubset(predef):
        print "only predefined symbols"
'
100000 loops, best of 3: 3.87 usec per loop
> python -m timeit -c '
import re
pat = re.compile("^[0x#$qi]*$")
my_string = "qwer123asd!@#$%^"
if pat.match(my_string):
        print("Only predefined symbols!")
'
100000 loops, best of 3: 5.15 usec per loop
>