How Can Regular Expressions Find Overlapping Matches?

How to find overlapping matches with regular expressions?

In the world of regular expressions, extracting overlapping matches is a unique challenge. Consider the example of the string "nnnn", we aim to identify all matches of "nn", including those that overlap. The desired output would be three matches:

<code>- nn**nn**
- n**nn**n
- nn**nn**</code>

Solution

Forward assertion

One way to solve this problem is to utilize forward assertions. The regular expression (?<=n)n matches the character 'n' that starts with another 'n'. This technique effectively identifies the end position of overlapping "nn" sequences:

<code>n**nn**nn
nn**nn**n
nn**nn**</code>

Reverse assertion

Alternatively, you can use reverse assertions. The regular expression (?=nn)n finds the character 'n' followed by another 'n'. This method focuses on the starting position of overlapping "nn" sequences:

<code>**nn**nn
n**nn**n
nn**nn**</code>

Enhanced reverse assertion using capturing groups

To further improve the reverse assertion method, we can combine it with capturing groups. The regular expression (n)(?=(n)) captures the first and second 'n' characters into two separate groups. This not only allows us to locate overlapping "nn" sequences, but also extract individual characters:

<code>**n**nn
n**n**n
nn**n**</code>

The benefit of this approach is its ability to adapt to complex regular expression patterns in reverse assertions and extract the corresponding matches via backreferences.

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