Get Second Occurrence of Substring in Python String

When we try to work in Python with the concepts of strings, this may create situations where you need to find out the second occurrence of a specific substring with a larger string. This may be achieved through out the different approaches as we know but we will try to explore Two different methods to do this.

Find The Second Occurrence Of A Substring In Python String

Below are some of the ways by which we can find the second occurrence of a substring in string in Python:

• Using find() function
• Using split() and List Slicing
• Using Regular Expression

Find The Second Occurrence Of A Substring Using find() method

In this example, the function `find_second_occurrence` is defined to find and return the index of the second occurrence of a specified substring in a given main string. The function checks for the first occurrence using `find()`, and if found, it looks for the second occurrence starting from the position after the first, providing the result or an error message if the substring appears only once or not at all.

Python3

def find_second_occurrence(main_str, sub_str):
    first_occurrence = main_str.find(sub_str)

    if first_occurrence != -1:
        second_occurrence = main_str.find(sub_str, first_occurrence + 1)
        return second_occurrence if second_occurrence != -1 else "Substring appears only once."
    else:
        return "Specified substring not found."


# Example usage
main_text = "Python is powerful, and Python is versatile."
sub_text = "Python"
result = find_second_occurrence(main_text, sub_text)
print("Index of the second occurrence:", result)

Output

Index of the second occurrence: 24

Time Complexity: O(n)
Space Complexity: O(1)

Find The Second Occurrence Of A Substring Using str.index() Method

In this example, the function `find_second_occurrence_index_v2` utilizes the `str.index()` method to find and return the index of the second occurrence of a specified substring in a given main string. It employs a try-except block to handle the case where the substring appears only once or not at all, providing the result or an appropriate error message.

Python3

def find_second_occurrence_index_v2(main_str, sub_str):
    try:
        first_occurrence = main_str.index(sub_str)
        second_occurrence = main_str.index(sub_str, first_occurrence + 1)
        return second_occurrence
    except ValueError:
        return "Substring appears only once or not at all."

# Example usage
main_text = "Python is powerful, and Python is versatile."
sub_text = "Python"
result_index_v2 = find_second_occurrence_index_v2(main_text, sub_text)
print("Index of the second occurrence:", result_index_v2)

Output

Index of the second occurrence: 24

Time Complexity: O(n)
Space Complexity: O(1)

Find The Second Occurrence Of A Substring Using RegularExpression

In this example, the function `find_second_occurrence_regex` utilizes the `re.finditer()` method to find all occurrences of the specified substring (`’Python’`) in a given main string. It then extracts the start positions of each match using a list comprehension. The example demonstrates finding the index of the second occurrence of the substring “Python” in the given main text using regular expressions.

Python3

import re

def find_second_occurrence_regex(main_str, sub_str):
    matches = [match.start() for match in re.finditer(sub_str, main_str)]

    if len(matches) > 1:
        return matches[1]
    else:
        return "Either the substring appears only once or not at all."

# Example usage
main_text = "Python is powerful, and Python is versatile."
sub_text = "Python"
result_regex = find_second_occurrence_regex(main_text, sub_text)
print("Index of the second occurrence:", result_regex)

Output

Index of the second occurrence: 24

Time Complexity: O(n)
Space Complexity: O(m)