Python 3 solution using only the exercise material, without any extra knowledge of re package.

import re

S = input() k = input()

i = 0

while i < len(S) - 1:

m = re.search(k, S[i:])

if not m:
    if i ==0:
        print((-1, -1))
    break

elif m:
    print((m.start()+i, m.end()+i-1))
    i += m.end() - 1 if len(k)!=1 else m.end()