Python | Get the starting index for all occurrences of given substring

Given a string and a substring, the task to find out the starting index for all the occurrences of a given substring in a string. Let’s discuss a few methods to solve the given task.

Method #1: Using Naive Method

# Python3 code to demonstrate
# to find all occurrences of substring in
# a string

# Initialising string
ini_string = 'xbzefdgstbzefzexezef'

# Initialising sub-string
sub_string = 'zef'

# Printing initial string and sub-string
print ("initial_strings : ", ini_string, "\nsubstring : ", sub_string)

res = []
flag = 0
k = 0

# Finding all occurrences of substring
# in a string using Naive method
for i in range(0, len(ini_string)):
    k = i
    flag = 0
    for j in range(0, len(sub_string)):
        if ini_string[k] != sub_string[j]:
            flag = 1
        if flag:
            break
        k = k + 1
    if flag == 0:
        res.append(i)


# printing result(
print ("resultant positions", str(res))

Output:

initial_strings :  xbzefdgstbzefzexezef 
substring :  zef
resultant positions [2, 10, 17]

&nsbp;
Method #2: Using list comprehension

# Python3 code to demonstrate
# to find all occurrences of substring in
# a string

# Initialising string
ini_string = 'xbzefdgstbzefzexezef'

# Initialising sub-string
sub_string = 'zef'

# Printing initial string and sub-string
print ("initial_strings : ", ini_string, "\nsubstring : ", sub_string)

res = []
# Finding all occurrences of substring
# in a string using list comprehension
res = [i for i in range(len(ini_string))
       if ini_string.startswith(sub_string, i)]

# printing result(
print ("resultant positions", str(res))

Output:

initial_strings :  xbzefdgstbzefzexezef 
substring :  zef
resultant positions [2, 10, 17]

Method #3: Using regex

# Python3 code to demonstrate
# to find all occurrences of substring in
# a string
import re

# Initialising string
ini_string = 'xbzefdgstbzefzexezef'

# Initialising sub-string
sub_string = 'zef'

# Printing initial string and sub-string
print ("initial_strings : ", ini_string, "\nsubstring : ", sub_string)

res = []
# Finding all occurrences of substring
# in a string using re.finditer
res = [m.start() for m in re.finditer(sub_string, ini_string)]

# printing result(
print ("resultant positions", str(res))

Output:

initial_strings :  xbzefdgstbzefzexezef 
substring :  zef
resultant positions [2, 10, 17]