Find all occurrences of a Substring in a String in Python

# Table of Contents

1. Find all indexes of a substring using startswith()
2. Find all indexes of a substring in a String using re.finditer()
3. Find all indexes of a substring in a String using a for loop
4. Find all indexes of a substring using a while loop
5. Finding only non-overlapping results

# Find all indexes of a substring using startswith()

To find all indexes of a substring in a string:

1. Use a list comprehension to iterate over a range object of the string's length.
2. Check if each character starts with the given substring and return the result.

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string = 'bobby hadz bobbyhadz.com'

indexes = [
    index for index in range(len(string))
    if string.startswith('bobby', index)
]

print(indexes)  # 👉️ [0, 11]

We used a range object to iterate over the string.

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string = 'bobby hadz bobbyhadz.com'

# [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,
#  12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23]
print(list(range(len(string))))

The range() class is commonly used for looping a specific number of times in for loops and takes the following arguments:

If you only pass a single argument to the range() constructor, it is considered to be the value for the stop parameter.

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for n in range(5):
    print(n) # 👉️ 0 1 2 3 4

result = list(range(5))

# 👇️ [0, 1, 2, 3, 4]
print(result)

On each iteration, we check if the slice of the string that starts at the current character starts with the given substring.

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string = 'bobby hadz bobbyhadz.com'

indexes = [
    index for index in range(len(string))
    if string.startswith('bobby', index)
]

print(indexes)  # 👉️ [0, 11]

If the condition is met, the corresponding index is returned.

The new list contains all of the indexes of the substring in the string.

# Find all indexes of a substring in a String using re.finditer()

This is a three-step process:

1. Use the re.finditer() to get an iterator object of the matches.
2. Use a list comprehension to iterate over the iterator.
3. Use the match.start() method to get the indexes of the substring in the string.

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import re

string = 'bobby hadz bobbyhadz.com'

indexes = [
    match.start() for match in re.finditer(r'bob', string)
]
print(indexes)  # 👉️ [0, 11]

The re.finditer() method takes a regular expression and a string and returns an iterator object containing the matches for the pattern in the string.

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import re

string = 'bobby hadz bobbyhadz.com'

# 👇️ [<re.Match object; span=(0, 3), match='bob'>,
# <re.Match object; span=(11, 14), match='bob'>]
print(list(
    re.finditer(r'bob', string)
))

The match.start() method returns the index of the first character of the match.

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import re

string = 'bobby hadz bobbyhadz.com'

print(
    list(re.finditer(r'bob', string))[0].start()  # 👉️ 0
)

print(
    list(re.finditer(r'bob', string))[1].start()  # 👉️ 11
)

The new list contains the index of all occurrences of the substring in the string.

Copied!
import re

string = 'bobby hadz bobbyhadz.com'

indexes = [
    match.start() for match in re.finditer(r'bob', string)
]
print(indexes)  # 👉️ [0, 11]

Alternatively, you can use a for loop.

# Find all indexes of a substring in a String using a for loop

This is a four-step process:

1. Declare a new variable that stores an empty list.
2. Use the re.finditer() to get an iterator object of the matches.
3. Use a for loop to iterate over the object.
4. Append the index of each match to the list.

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import re

string = 'bobby hadz bobbyhadz.com'

indexes = []

for match in re.finditer(r'bob', string):
    indexes.append(match.start())

print(indexes)  # 👉️ [0, 11]

We used a for loop to iterate over the iterator object.

On each iteration, we use the match.start() method to get the index of the current match and append the result to the indexes list.

The list.append() method adds an item to the end of the list.

# Find all indexes of a substring using a while loop

You can also use a while loop to find all indexes of a substring in a string.

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def find_indexes(a_string, substring):
    start = 0

    indexes = []

    while start < len(a_string):
        start = a_string.find(substring, start)

        if start == -1:
            return indexes

        indexes.append(start)

        start += 1

    return indexes


string = 'bobby hadz bobbyhadz.com'
print(find_indexes(string, 'bob'))  # 👉️ [0, 11]

string = 'bobobobob'
print(find_indexes(string, 'bob'))  # 👉️ [0, 2, 4, 6]

The find_indexes substring takes a string and a substring and returns a list containing all of the indexes of the substring in the string.

We used a while loop to iterate for as long as the start variable is less than the string's length.

On each iteration, we use the str.find() method to find the next index of the substring in the string.

The str.find method returns the index of the first occurrence of the provided substring in the string.

The method returns -1 if the substring is not found in the string.

If the substring is not found in the string -1 is returned and we return the indexes list.

Otherwise, we add the index of the occurrence to the list and increment the start variable by 1.

Notice that the function in the example finds indexes of overlapping substrings as well.

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def find_indexes(a_string, substring):
    start = 0

    indexes = []

    while start < len(a_string):
        start = a_string.find(substring, start)

        if start == -1:
            return indexes

        indexes.append(start)

        start += 1

    return indexes

string = 'bobobobob'
print(find_indexes(string, 'bob'))  # 👉️ [0, 2, 4, 6]

# Finding only non-overlapping results

If you need to only find the indexes of the non-overlapping substrings, add the length of the substring to the start variable.

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def find_indexes(a_string, substring):
    start = 0

    indexes = []

    while start < len(a_string):
        start = a_string.find(substring, start)

        if start == -1:
            return indexes

        indexes.append(start)

        start += len(substring) # 👈️ only non-overlapping

    return indexes


string = 'bobobobob'
print(find_indexes(string, 'bob'))  # 👉️ [0, 4]

Instead of adding 1 to the start variable when iterating, we added the length of the substring to only get the indexes of the non-overlapping matches.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• Find the index of Elements that meet a condition in Python
• Find a common substring between two strings in Python
• Find common values in multiple Lists in Python
• Find elements in one List that are not in the other (Python)