How Can I Get Overlapping Matches with Regular Expressions and the Global Flag?

Overlapping String Matching with Regular Expressions

When using a regular expression with the global flag (/g) to match a string, you may encounter cases where overlapping matches are not returned as expected. For instance, in the string "12345," matching using /d{3}/g only yields the match "123" instead of ["123", "234", "345"].

The reason for this behavior lies in the nature of regex matching with the global flag. The regex engine reads and consumes the matched substring, advancing its index to the position immediately following the match. In this case, after matching "123," the index moves to position 4, leaving only "45" for subsequent matching.

Techniques for Overlapping Matching

To achieve overlapping matches, several approaches can be employed:

• Zero-Width Assertions:

  • Utilize a positive lookahead with a capturing group to test all positions in the input string.
  • Manually advance the RegExp.lastIndex to avoid infinite loops.
  • Supported by .NET, Python, PHP, Ruby, and Java (using matchAll).

Example (JavaScript using matchAll):

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

This code uses a positive lookahead to test for all positions in the string that match three consecutive digits. The result is an array of the matched substrings, including ["123", "234", "345"].

Additional Considerations:

• Zero-width assertions are only applicable when you need to match overlapping substrings.
• Some regex engines have built-in support for overlapping matches (e.g., PCRE in PHP and Ruby).
• Alternative approaches may include using loops with substrings or split functions.

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