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ECMAScript RegExp Match Indices
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syg and rbuckton Make .indices a data property. Closes #29. (#31)
* Make .indices a data property. Closes #29.

Making .indices a caching getter in the spec makes the function identity
of the getter observable. This makes it necessary for implementations to
create a new Function object per result object of
`RegExp.prototype.exec`. The intended laziness of .indices is possible
in implementations even with it as a data property in spec.

* Use CreateDataProperty instead of DefinePropertyOrThrow for "indices"
Latest commit adb39a5 Aug 22, 2019

RegExp Match Indices for ECMAScript

ECMAScript RegExp Match Indicies provide additional information about the start and end indices of captured substrings relative to the start of the input string.

NOTE: This proposal was previously named "RegExp Match Array Offsets", but has been renamed to more accurately represent the current status of the proposal.


Stage: 3 Champion: Ron Buckton (@rbuckton)

For detailed status of this proposal see TODO, below.


  • Ron Buckton (@rbuckton)


Today, ECMAScript RegExp objects can provide information about a match when calling the exec method. This result is an Array containing information about the substrings that were matched, along with additional properties to indicate the input string, the index in the input at which the match was found, as well as a groups object containing the substrings for any named capture groups.

However, there are several more advanced scenarios where this information may not necessarily be sufficient. For example, an ECMAScript implementation of TextMate Language syntax highlighting needs more than just the index of the match, but also the start and end indices for individual capture groups.

As such, we propose the adoption of an additional indices property on the array result (the substrings array) of RegExp.prototype.exec(). This property would itself be an indices array containing a pair of start and end indices for each captured substring. Any unmatched capture groups would be undefined, similar to their corresponding element in the substrings array. In addition, the indices array would itself have a groups property containing the start and end indices for each named capture group.

Prior Art


const re1 = /a+(?<Z>z)?/;

// indices are relative to start of the input string:
const s1 = "xaaaz";
const m1 = re1.exec(s1);
m1.indices[0][0] === 1;
m1.indices[0][1] === 5;
s1.slice(...m1.indices[0]) === "aaaz";

m1.indices[1][0] === 4;
m1.indices[1][1] === 5;
s1.slice(...m1.indices[1]) === "z";

m1.indices.groups["Z"][0] === 4;
m1.indices.groups["Z"][1] === 5;
s1.slice(...m1.indices.groups["Z"]) === "z";

// capture groups that are not matched return `undefined`:
const m2 = re1.exec("xaaay");
m2.indices[1] === undefined;
m2.indices.groups["Z"] === undefined;


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].
    • V8 v7.8, behind the --harmony-regexp-match-indices flag (tracking bug)
  • A pull request has been sent to tc39/ecma262 with the integrated spec text.
  • The ECMAScript editor has signed off on the pull request.
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