`regex-recursion`

This is an extension for the regex template tag that adds support for recursive patterns up to a specified max depth N, where N must be 2–100.

You can add recursion to a regex pattern via one of the following:

(?R=N) — Recursively match the entire pattern at this position.
\g<name&R=N> — Recursively match the contents of group name at this position. The \g subroutine must be used within the referenced group.

Any backreferences are unique per depth level.

Examples

Match an equal number of two different patterns:

import {rregex} from 'regex-recursion';

// Matches sequences of up to 50 'a' chars followed by the same number of 'b'
rregex`a(?R=50)?b`.exec('test aaaaaabbb')[0];
// → 'aaabbb'

Match an equal number of two different patterns, as the whole string:

const re = rregex`
  ^
  (?<balanced>
    a
    # Recursively match just the specified group
    \g<balanced&R=50>?
    b
  )
  $
`;
re.test('aaabbb'); // → true
re.test('aaabb'); // → false

Match balanced parentheses:

// Matches all balanced parentheses up to depth 10
const parens = rregex('g')`\(
  ( [^\(\)] | (?R=10) )*
\)`;

'test (balanced ((parens))) ) () ((a)) ((b)'.match(parens);
// → ['(balanced ((parens)))', '()', '((a))', '(b)']

Match palindromes:

const palindromes = rregex('gi')`(?<char>\w) ((?R=10)|\w?) \k<char>`;
// Palindrome maxlength: 21 = 2 chars (left+right) × depth 10 + 1 in the center

'Racecar, ABBA, and redivided'.match(palindromes);
// → ['Racecar', 'ABBA', 'edivide']

Match palindromes as complete words:

const palindromeWords = rregex('gi')`
  \b
  (?<palindrome>
    (?<char>\w)
    # Recurse, or match a lone unbalanced char in the center
    ( \g<palindrome&R=10> | \w? )
    \k<char>
  )
  \b
`;

'Racecar, ABBA, and redivided'.match(palindromeWords);
// → ['Racecar', 'ABBA']

Sugar free

Template tag rregex is sugar for applying recursion support via a postprocessor with tag regex. You can also add recursion support the verbose way:

import {regex} from 'regex';
import {recursion} from 'regex-recursion';

regex({flags: 'g', postprocessors: [recursion]})`a(?R=2)?b`;

Install and use

npm install regex-recursion

import {rregex} from 'regex-recursion';

In browsers:

<script src="https://cdn.jsdelivr.net/npm/regex-recursion/dist/regex-recursion.min.js"></script>
<script>
  // Recommended
  const {rregex} = Regex.ext;
</script>