# 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]
```

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