How Does `re.findall()` Handle Overlapping Matches in Regular Expressions?

Understanding Regex Overlapping Matches

When using re.findall() to match regular expressions, understanding how overlapping matches are handled is crucial. By default, re.findall() returns non-overlapping matches.

Case Study: hello and ww

Consider the following pattern:

>>> match = re.findall(r'\w\w', 'hello')
>>> print match
['he', 'll']

As expected, this pattern matches two-character sequences. However, it does not match 'el' or 'lo' because they overlap with 'he' and 'll,' respectively.

Overlapping Matches with Lookahead Assertions

To find overlapping matches, a lookahead assertion can be employed. A lookahead assertion, denoted by (?=...), verifies if a specified pattern exists next without consuming the input string.

Using this concept, the following expression successfully captures both overlapping and non-overlapping matches:

>>> re.findall(r'(?=(\w\w))', 'hello')
['he', 'el', 'll', 'lo']

The pattern now reads: "find any location where a two-character word follows." This ensures that all possible matching sequences, including both overlapping and non-overlapping ones, are captured.

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