Python - All occurrences of substring in string

A substring is a contiguous occurrence of characters within a string. Identifying all instances of a substring is important for verifying various tasks. In this article, we will check all occurrences of a substring in String.

Using re.finditer()

re.finditer() returns an iterator yielding match objects for all non-overlapping matches of a pattern in a string that allows to check for specific patterns, such as digits, throughout the string.

import re

# Define the input string and substring
s = "hello world, hello universe"
substring = "hello"

# Find all occurrences using re.finditer
positions = [match.start() for match in re.finditer(substring, s)]

print(positions)

[0, 13]

Explanation:

• Use re.finditer() to find matches: The re.finditer() function searches for all occurrences of the substring "hello" in the string "hello world, hello universe", returning an iterator of match objects.
• Extract start positions: A list comprehension is used to extract the starting position of each match using match.start().

Using str.find() in a loop

Using str.find() in a loop allows to find all occurrences of a substring by repeatedly searching for the next match starting from the last found index. The loop continues until no more matches are found (when find() returns -1).

s = "hello world, hello universe"
substring = "hello"

# Find all occurrences using str.find in a loop
positions = []

start = 0
while True:
    start = s.find(substring, start)
    if start == -1:
        break
    positions.append(start)
    start += len(substring)

print(positions)

[0, 13]

Explanation

• Use find() in a loop: The find() method is called repeatedly, starting from the last found position, to locate each occurrence of the substring "hello" in the string "hello world, hello universe".
• Track positions: Each found position is added to the positions list, and the start index is updated to move past the current match to continue searching for subsequent occurrences.

Using List Comprehension with range():

List comprehension with range() can be used to generate all starting positions of a substring by iterating over the string indices. It checks each possible position within the string to see if the substring matches starting from that index.

s = "hello world, hello universe"
substring = "hello"

# Find all occurrences using list comprehension
positions = [i for i in range(len(s)) if s.startswith(substring, i)]

print(positions)  

[0, 13]

Explanation

• Use list comprehension with startswith(): The list comprehension iterates over each index i in the string, checking if the substring "hello" starts at that position using text.startswith(substring, i).
• Store starting positions: If the substring matches at index i, that index is added to the positions list.