regex-recursion

This is a plugin for the regex library that adds support for recursive matching up to a specified max depth N, where N must be between 2 and 100. Generated regexes are native RegExp instances, and support all JavaScript regular expression features.

Recursive matching is added to a regex via one of the following:

• (?R=N) — Recursively match the entire regex 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/backreferences, and makes them independent per depth level. So e.g. groups.name on a match object is the value captured by group name at the top level of the recursion stack.

Install and use

npm install regex regex-recursion

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

const re = regex({plugins: [recursion]})`…`;

In browsers (using a global name):

<script src="https://cdn.jsdelivr.net/npm/regex@4.0.0/dist/regex.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/regex-recursion@4.0.0/dist/regex-recursion.min.js"></script>
<script>
  const {regex} = Regex;
  const {recursion} = Regex.plugins;

  const re = regex({plugins: [recursion]})`…`;
</script>

Examples

Match an equal number of two different subpatterns

Anywhere within a string

// Matches sequences of up to 50 'a' chars followed by the same number of 'b'
const re = regex({plugins: [recursion]})`a(?R=50)?b`;
re.exec('test aaaaaabbb')[0];
// → 'aaabbb'

As the entire string

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

Note the ^ and $ anchors outside of the recursive subpattern.

Match balanced parentheses

// Matches all balanced parentheses up to depth 50
const parens = regex({flags: 'g', plugins: [recursion]})`\(
  ( [^\(\)] | (?R=50) )*
\)`;

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

Following is an alternative that matches the same strings, but adds a nested quantifier. It then uses an atomic group to prevent this nested quantifier from creating the potential for catastrophic backtracking.

const parens = regex({flags: 'g', plugins: [recursion]})`\(
  ( (?> [^\(\)]+ ) | (?R=50) )*
\)`;

This matches sequences of non-parens in one step with the nested + quantifier, and avoids backtracking into these sequences by wrapping it with an atomic group (?>…). Given that what the nested quantifier + matches overlaps with what the outer group can match with its * quantifier, the atomic group is important here. It avoids exponential backtracking when matching long strings with unbalanced parens.

Atomic groups are provided by the base regex library.

Match palindromes

Match palindroms anywhere within a string

const palindromes = regex({flags: 'gi', plugins: [recursion]})`
  (?<char> \w )
  # Recurse, or match a lone unbalanced char in the middle
  ( (?R=15) | \w? )
  \k<char>
`;

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

In the example above, the max length of matched palindromes is 31. That's because it sets the max recursion depth to 15 with (?R=15). So, depth 15 × 2 chars (left + right) for each depth level + 1 optional unbalanced char in the middle = 31. To match longer palindromes, the max recursion depth can be increased to a max of 100, which would enable matching palindromes up to 201 characters long.

Match palindromes as complete words

const palindromeWords = regex({flags: 'gi', plugins: [recursion]})`\b
  (?<palindrome>
    (?<char> \w )
    ( \g<palindrome&R=15> | \w? )
    \k<char>
  )
\b`;

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

Note the \b word boundaries outside of the recursive subpattern.