How Can Regular Expressions Effectively Handle Overlapping Matches Like 'nn' in 'nnnn'?

Overlap matching in regular expressions

In the world of regular expressions, the concept of overlapping matching can be difficult to handle effectively. Let's explore this issue with a concrete example.

Problem Statement:

Consider a string like "nnnn". The goal is to identify all occurrences of "nn" while taking overlap into account. In other words, the desired output would be three matches:

<code>nnnnn
n nnn
nn nnn</code>

Solution 1: Forward assertion

One possible solution involves using forward assertions:

<code>(?<=n)nn</code>

This expression ensures that a match is preceded by an occurrence of "n". It returns the ending position of each "nn" substring.

Solution 2: Negative assertion

A more intuitive approach is to use negative assertions:

<code>(?=nn)</code>

This expression checks whether "nn" follows the current position. However, it does not capture the actual "nn" sequence.

Improved solution: combine forward assertions and capturing groups

To capture overlapping "nn" substrings, we can combine forward assertions with capturing groups:

<code>(n)(?=(n))</code>

This expression captures the first "n" in group(1) and the next "n" in group(2). By using a capturing group we can access the actual matched substring.

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