How Can I Find Overlapping Matches in a String Using Regular Expressions?

Understanding Overlapping String Matching with Regex

In certain scenarios, the string#match method with the global flag may not match overlapping sequences in the input string. For instance, consider the string "12345". Using the regular expression /d{3}/g, we expect to obtain three matches: ["123", "234", "345"]. However, we only get a single match, "123".

This is because the string#match method consumes (i.e., reads and advances its index past) the matched characters. As such, after matching "123", the current index points to '4', and the regex engine stops, leaving no more matches to be found.

Solving Overlapping Matches with a Zero-Width Assertion

To address this limitation, a popular technique involves using a positive lookahead assertion with a capturing group. This approach asserts the presence of a substring without actually consuming it. By repeatedly testing all positions in the input string, we can capture the desired overlapping matches.

var re = /(?=(\d{3}))/g;
console.log(Array.from('12345'.matchAll(re), x => x[1]);

In this example, we create a regular expression with a positive lookahead assertion that captures three consecutive digits without consuming them. By iterating through the input string using matchAll, we obtain the desired list of matches: ["123", "234", "345"].

This technique is supported in various programming languages, empowering developers to handle overlapping string matching scenarios with ease.

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