Find Index of Last Substring Occurrence in Python String

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Problem Formulation

Given a string and a substring in Python. How to find the index of the last occurrence of the substring in Python?

Let’s have a look at a couple of examples to thoroughly understand the problem:

Example 1:
string = 'fifi'
substring = 'fi'
result: 2

Example 2:
string = 'hello'
substring = 'l'
result: 3

Example 3:
string = 'finxter finxter finxter'
substring = 'finxter'
result: 16

Let’s dive into the first and most Pythonic method next!

Method 1: rfind()

The Python string.rfind(substr) method returns the highest index in the string where a substring is found. Thus, it finds the index of the last occurrence of the substring in a given string or returns -1 if not found.

Here’s an example:

string = 'fifi'
substring = 'fi'
print(string.rfind(substring))
# 2

For comprehensibility, let’s dive into the other two examples introduced in the problem formulation:

string = 'hello'
substring = 'l'
print(string.rfind(substring))
# 3


string = 'finxter finxter finxter'
substring = 'finxter'
print(string.rfind(substring))
# 16

You can find some background information on rfind() and multiple other string methods in the following video—conquer string methods once and for all! 🙂

Method 2: string.rindex()

The Python string.rindex(substr) method returns the highest index in the string where a substring is found. Thus, it finds the index of the last occurrence of the substring in a given string or raises a ValueError if not found.

Let’s have an analogous example:

string = 'fifi'
substring = 'fi'
print(string.rindex(substring))
# 2


string = 'hello'
substring = 'l'
print(string.rindex(substring))
# 3


string = 'finxter finxter finxter'
substring = 'finxter'
print(string.rindex(substring))
# 16

The difference between rfind() and rindex() is that while both return the index of the last occurrence of the substring, the former returns -1 and the latter raises a ValueError if the substring doesn’t exist in the string.

Method 3: Regex Search Negative Lookahead

Just for fun, here’s a regular expression that finds the index of the last occurrence of a substring in a given string:

re.search(substring + '(?!.*' + substring + ')', string).start()

Before I’ll explain it to you, let’s have a look at whether it really does what it is supposed to do! 🙂

import re


string = 'fifi'
substring = 'fi'
print(re.search(substring + '(?!.*' + substring + ')', string).start())
# 2


string = 'hello'
substring = 'l'
print(re.search(substring + '(?!.*' + substring + ')', string).start())
# 3


string = 'finxter finxter finxter'
substring = 'finxter'
print(re.search(substring + '(?!.*' + substring + ')', string).start())
# 16

Okay, it works—but why?

You need to understand a couple of regular expression concepts for this:

  • The re.search(pattern, string) method finds the first occurrence of the pattern in the given string.
  • We use the pattern substring + '(?!.*' + substring + ')' that finds the first occurrence of the substring for which no other occurrence of substring follows in the string. This is the last occurrence of substring in the string.
  • The expression (?! ... ) is called negative lookahead and it makes sure that the pattern within it does not follow immediately afterwards (no/negative match).
  • The return value of re.search() is a match object on which we can call the .start() method to obtain the index of the match in the original string.

You can learn more about the regex search method in this video tutorial: