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https://github.com/tc39/proposal-regexp-modifiers

Regular Expression Pattern Modifiers for ECMAScript
https://github.com/tc39/proposal-regexp-modifiers

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Regular Expression Pattern Modifiers for ECMAScript

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# Regular Expression Pattern Modifiers for ECMAScript



## Status



**Stage:** 3  

**Champion:** Ron Buckton ([@rbuckton](https://github.com/rbuckton))  



_For detailed status of this proposal see [TODO](#todo), below._  



## Authors



* Ron Buckton ([@rbuckton](https://github.com/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 



* [Perl](https://rbuckton.github.io/regexp-features/engines/perl.html#feature-modifiers)  

* [PCRE](https://rbuckton.github.io/regexp-features/engines/pcre.html#feature-modifiers)  

* [Boost.Regex](https://rbuckton.github.io/regexp-features/engines/boost.regex.html#feature-modifiers)  

* [.NET](https://rbuckton.github.io/regexp-features/engines/dotnet.html#feature-modifiers)  

* [Oniguruma](https://rbuckton.github.io/regexp-features/engines/oniguruma.html#feature-modifiers)  

* [Hyperscan](https://rbuckton.github.io/regexp-features/engines/hyperscan.html#feature-modifiers)  

* [ICU](https://rbuckton.github.io/regexp-features/engines/icu.html#feature-modifiers)  

* [Glib/GRegex](https://rbuckton.github.io/regexp-features/engines/glib-gregex.html#feature-modifiers)  



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](https://github.com/tc39/proposal-regexp-modifiers/issues/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



```js

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](https://1drv.ms/p/s!AjgWTO11Fk-Tkfl7c6yR-2P8T4gn0w?e=cvaUL2))

  - Outcome: Advanced to Stage 1

- December 15th, 2021 — Proposed for Stage 2 ([slides](https://1drv.ms/p/s!AjgWTO11Fk-Tkfs3yIyrh3hZ2k6PCQ?e=Yodx4H))

  - 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](https://1drv.ms/p/s!AjgWTO11Fk-Tkf5daRnRsxu8BY5Nsg?e=UKVf8W))

  - Outcome: [Advanced to Stage 3](https://github.com/tc39/notes/blob/31edb829db604fdb0255b21238b20898b66cee41/meetings/2022-06/jun-07.md)



# TODO



The following is a high-level list of tasks to progress through each stage of the [TC39 proposal process](https://tc39.github.io/process-document/):



### Stage 1 Entrance Criteria



* [x] Identified a "[champion][Champion]" who will advance the addition.  

* [x] [Prose][Prose] outlining the problem or need and the general shape of a solution.  

* [x] Illustrative [examples][Examples] of usage.  

* [ ] ~~High-level [API][API].~~  



### Stage 2 Entrance Criteria



* [x] [Initial specification text][Specification].  

* [ ] ~~[Transpiler support][Transpiler] (_Optional_)~~.  



### Stage 3 Entrance Criteria



* [x] [Complete specification text][Specification].  

* [x] Designated reviewers have [signed off][Stage3ReviewerSignOff] on the current spec text (1 of 2).  

* [x] The ECMAScript editor has [signed off][Stage3EditorSignOff] on the current spec text.  



### Stage 4 Entrance Criteria



* [ ] [Test262](https://github.com/tc39/test262) acceptance tests have been written for mainline usage scenarios and [merged][Test262PullRequest].  

  * [x] written  

  * [ ] merged  

* [ ] Two compatible implementations which pass the acceptance tests:  

  * [X] [V8][Implementation1] - Shipping in [Chrome 125](https://developer.chrome.com/release-notes/125)/Edge 125

  * [ ] [SpiderMonkey][Implementation2] - In Progress

  * [ ] JSC  

  * [ ] [Engine262](https://github.com/engine262/engine262/pull/229)  

* [x] A [pull request][Ecma262PullRequest] has been sent to tc39/ecma262 with the integrated spec text.  

* [ ] The ECMAScript editor has signed off on the [pull request][Ecma262PullRequest].  

  * [x] [Kevin Gibbons](https://github.com/tc39/ecma262/pull/3221#pullrequestreview-1735554031)  

  * [ ] [Michael Ficarra](https://github.com/tc39/ecma262/pull/3221#pullrequestreview-1784954743)  



[Process]: https://tc39.es/process-document/

[Proposals]: https://github.com/tc39/proposals/

[Grammarkdown]: http://github.com/rbuckton/grammarkdown#readme

[Champion]: #status

[Prose]: #motivations

[Examples]: #examples

[API]: #api

[Specification]: https://rbuckton.github.io/proposal-regexp-modifiers



[Transpiler]: #todo

[Stage3ReviewerSignOff]: #todo

[Stage3EditorSignOff]: #todo

[Test262PullRequest]: https://github.com/tc39/test262/pull/3960

[Implementation1]: https://bugs.chromium.org/p/v8/issues/detail?id=12956

[Implementation2]: https://bugzilla.mozilla.org/show_bug.cgi?id=1899813

[Ecma262PullRequest]: https://github.com/tc39/ecma262/pull/3221