`regex-recursion`

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

Recursive matching is added 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 called within the referenced group.

Recursive matching supports named captures and backreferences, and makes them independent per depth level. So e.g. groups.name on a RegExp match array is the value captured by group name at the top level of the recursion stack.

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 entire string:

import {rregex} from 'regex-recursion';

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:

import {rregex} from 'regex-recursion';

// Matches all balanced parentheses up to depth 50
const parens = rregex('g')`\(
  ( [^\(\)] | (?R=50) )*
\)`;

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

// ----------
// Here's an alternative that matches the same strings
const parens = rregex('g')`\(
  ( (?> [^\(\)]+ ) | (?R=50) )*
\)`;
// This matches sequences of non-parens in one step with the `+` quantifier,
// and avoids backtracking into these sequences by using an atomic group
// `(?>…)`. Given the nested quantifier, the atomic group is important here.
// It avoids runaway backtracking when matching long strings with unbalanced
// parens. Atomic groups are provided by the base `regex` package

Match palindromes:

import {rregex} from 'regex-recursion';

const palindromes = rregex('gi')`(?<char>\w) ((?R=15)|\w?) \k<char>`;
// Palindrome max length: 31 = 2 chars (left + right) × depth 15 + 1 in center

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

Match palindromes as complete words:

import {rregex} from 'regex-recursion';

const palindromeWords = rregex('gi')`\b
  (?<palindrome>
    (?<char> \w )
    # Recurse, or match a lone unbalanced char in the center
    ( \g<palindrome&R=15> | \w? )
    \k<char>
  )
\b`;

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

Sugar free

Template tag rregex is sugar for using the base regex tag and adding recursion support via a postprocessor. You can also add recursion support the verbose way:

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

regex({flags: 'i', 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>
  const {rregex} = Regex.ext;
</script>