Why Doesn't `re.findall()` Return Overlapping Regex Matches, and How Can I Fix It?

Digging Deeper into Overlapping Regex Matches

In the world of regular expressions, understanding why certain matches might not be found can be puzzling. Consider the following example:

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

As expected, this snippet returns a list containing 'he' and 'll', which match the pattern of two-letter sequences. However, one might wonder why 'el' and 'lo', which also seem to fit the pattern, are not included in the result.

This behavior stems from the default behavior of the re.findall function, which does not produce overlapping matches. In other words, it moves along the string, finding only the first match for a given pattern, and not considering any potential overlaps.

To address this issue, there is a clever workaround involving lookahead assertions. A lookahead assertion (?=...) matches a pattern without actually consuming any of the string. This allows us to find all overlapping matches that satisfy the given pattern.

For instance, to find all two-letter sequences in the string 'hello' using a lookahead assertion, the following expression can be used:

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

This expression will return a list containing ['he', 'el', 'll', 'lo'], as each two-letter sequence is successfully matched without overlap.

Understanding lookahead assertions and their practical applications can greatly enhance the effectiveness of regular expressions for complex matching scenarios.

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