Regular Expression Pattern Modifiers for ECMAScript


Stage: 1
Champion: Ron Buckton (@rbuckton)

For detailed status of this proposal see TODO, below.



One common capability amongst the majority of regular expression engines that is commonly used by parsers, syntax highlighters, and other tools is the capability to control a subset of regular expression flags such as:

Modifiers are especially helpful when regular expressions are defined in a context where executable code cannot be evaluated, such as a JSON configuration file or TextMate tmLanguage grammar file.

As part of this proposal, we will investigate each existing (and future-proposed) RegExp flag to determine whether they are feasible to used as modifiers.

Prior Art

See for additional information.


Modifiers allow you to change the currently active RegExp flags within a subexpression.

  • (?imsx-imsx) — Sets or unsets (using -) the specified RegExp flags starting at the current position until the next closing ) or the end of the pattern.
  • (?imsx-imsx:subexpression) — Sets or unsets (using -) the specified RegExp flags for the subexpression.

NOTE: Certain flags cannot be modified mid-expression. These currently include g (global), y (sticky), and d (hasIndices).

NOTE: The actual supported flags will be determined on a case-by-case basis.

NOTE: This has no conflicts with existing syntax, as ECMAScript currently produces an error for this syntax in both u and non-u modes.


const re1 = /^(?i)[a-z](?-i)[a-z]$/;
re1.test("ab"); // true
re1.test("Ab"); // true
re1.test("aB"); // false

const re2 = /^(?i:[a-z](?-i:[a-z]))$/;
re2.test("ab"); // true
re2.test("Ab"); // true
re2.test("aB"); // false


  • October 27th, 2021 — Proposed for Stage 1 (slides)
    • Outcome: Advanced to Stage 1


The following is a high-level list of tasks to progress through each stage of the TC39 proposal process:

Stage 1 Entrance Criteria

  • Identified a "champion" who will advance the addition.
  • Prose outlining the problem or need and the general shape of a solution.
  • Illustrative examples of usage.
  • High-level API.

Stage 2 Entrance Criteria

Stage 3 Entrance Criteria

Stage 4 Entrance Criteria

  • Test262 acceptance tests have been written for mainline usage scenarios and merged.
  • Two compatible implementations which pass the acceptance tests: [1], [2].
  • A pull request has been sent to tc39/ecma262 with the integrated spec text.
  • The ECMAScript editor has signed off on the pull request.