Awesome

Regular Expression Pattern Modifiers for ECMAScript

Status

Stage: 3
Champion: Ron Buckton (@rbuckton)

For detailed status of this proposal see TODO, below.

Authors

Ron Buckton (@rbuckton)

Motivations

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:

i — Ignore Case
m — Multiline
s — Single-line (a.k.a. "dot all")
x — Extended mode (see https://github.com/rbuckton/proposal-regexp-x-mode)

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 https://rbuckton.github.io/regexp-features/features/modifiers.html for additional information.

Syntax

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

(?imsx-imsx:subexpression) — Sets or unsets (using -) the specified RegExp flags for the 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.~~

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

NOTE: The actual supported flags will be determined on a case-by-case basis. See #1.

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

NOTE: The "self-bounded" form ((?imsx-imsx:subexpression)) advanced to Stage 2 on December 15th, 2021.

NOTE: The "unbounded" form ((?imsx-imsx)) is no longer being considered as part of this proposal as of December 15th, 2021.

Examples

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

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

History

October 27th, 2021 — Proposed for Stage 1 (slides)
- Outcome: Advanced to Stage 1
December 15th, 2021 — Proposed for Stage 2 (slides)
- Outcome: Advanced to Stage 2 with "self-bounded" form only ("unbounded" form did not advance).
- Stage 2 Reviewers: Richard Gibson, Waldemar Horwat
June 7th, 2022 — Proposed for Stage 3 (slides)
- Outcome: Advanced to Stage 3

TODO

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

Initial specification text.
~~Transpiler support (Optional)~~.

Stage 3 Entrance Criteria

Complete specification text.
Designated reviewers have signed off on the current spec text (1 of 2).
The ECMAScript editor has signed off on the current spec text.

Stage 4 Entrance Criteria