How Can I Find All Occurrences of a Substring in Python?

Finding All Occurrences of a Substring in Python

In Python, you can use the string.find() and string.rfind() methods to retrieve the index of a substring within a larger string. However, there isn't a built-in function specifically designed to return all occurrences of a substring.

Using Regular Expressions

A more powerful approach to finding multiple substring occurrences is to employ regular expressions:

import re

# Sample string
string = "test test test test"

# Find all occurrences of "test"
matches = [m.start() for m in re.finditer('test', string)]
print(matches)  # Output: [0, 5, 10, 15]

re.finditer generates a generator that yields individual match objects. Each match object provides the starting index of a matching substring.

Considering Overlapping Matches

By default, re.finditer looks for non-overlapping matches. To find overlapping matches, use a positive lookahead:

matches = [m.start() for m in re.finditer('(?=tt)', 'ttt')]
print(matches)  # Output: [0, 1]

The expression (?=tt) asserts that the substring "tt" appears at the current position but doesn't consume it.

Reverse Find-All Without Overlaps

To perform a reverse find-all without overlapping matches, combine positive and negative lookahead:

search = 'tt'
matches = [m.start() for m in re.finditer('(?=%s)(?!.{1,%d}%s)' % (search, len(search)-1, search), 'ttt')]
print(matches)  # Output: [1]

This expression ensures that "tt" appears immediately after the cursor but not within a certain lookback range (len(search)-1) in the reverse direction.

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