How Can I Match Overlapping Strings Using Regular Expressions?

Matching Overlapping Strings with Regular Expressions

When matching overlapping strings with a regex, it's important to understand how the match method and regular expression buffering work.

The Issue: Consuming Matched Characters

By default, the match method with a global flag (g) in a regular expression consumes matched characters and advances the index to the position right after the matched substring. For example, the regex /d{3}/g will match a sequence of three digits, but it will only return one match because it consumes the matched characters.

Solution: Zero-Width Assertions

To match overlapping strings, you can use a zero-width assertion, which is a positive lookahead with a capturing group. This assertion tests all positions in the input string, and if a match is found, the capturing group captures the matching substring. The RegExp.lastIndex property is then manually advanced to avoid infinite looping.

Using 'matchAll'

If your programming language supports the matchAll method, you can use it to implement zero-width assertions and find all overlapping matches. For instance, in JavaScript, the following code uses matchAll to find all overlapping three-digit substrings in the string "12345":

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

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