how to find the indexes of a substring in a string in python

import re
# matches_position_start will be a list of starting index positions
matches_start = re.finditer(word.lower(), string.lower())
matches_position_start = [match.start() for match in matches_start]

# matches_position_end will be a list of ending index positions
matches_end = re.finditer(word.lower(), string.lower())
matches_position_end = [match.end() for match in matches_end]

index = [idx for idx, s in enumerate(l) if 'tiger' in s][0]

def index_containing_substring(the_list, substring):
    for i, s in enumerate(the_list):
        if substring in s:
              return i
    return -1