How Can I Find Overlapping Matches Using Regular Expressions in Python?

Understanding Overlapping Matches with Regular Expressions

When using regular expressions, it's important to consider how overlapping matches are handled. In Python, the default behavior of re.findall is to return only non-overlapping matches.

Default Behavior: Non-Overlapping Matches

For instance, consider the expression:

>>> match = re.findall(r'\w\w', 'hello')

As expected, the match variable contains ['he', 'll']. The expression matches two consecutive characters at a time, resulting in non-overlapping matches.

Overlapping Matches with Lookahead Assertions

To find overlapping matches, you can use lookahead assertions. The syntax for a lookahead assertion is:

(?=...)

where ... represents a regular expression that matches the desired substring.

For example, the expression:

>>> re.findall(r'(?=(\w\w))', 'hello')

returns ['he', 'el', 'll', 'lo']. The lookahead assertion (?=(ww)) checks if the next two characters in the string match the pattern ww, without consuming them. Since every consecutive pair of characters satisfies this criterion, all possible overlapping matches are found.

Conclusion

By leveraging lookahead assertions, you can easily find overlapping matches with regular expressions. This is a powerful technique for tasks such as extracting substrings or validating input strings.

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