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ECMAScript RegExp match array offsets
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RegExp match array offsets for ECMAScript

ECMAScript RegExp match array offsets provide additional information about the position of a captured substring relative to the index of the match.

An example implementation can be found in regexp-measure.

NOTE: regexp-measure was built around the Stage 0 proposal and is no longer up to date with respect to the current proposed API design.


Stage: 1
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 offsets for individual capture groups. We also have no mechanism to indicate whether a capture group was merely empty vs. unmatched (either optional or in an unchosen alternative of a disjunction).

As such, we propose the addition of an optional second argument to exec that would take function callback that could be used to map the offsets of each capture to be used as the result in the resulting match array. This callback would be supplied with three arguments: The start position of the match within the input (-1 if unmatched), the end position of the match within the input (-1 if unmatched), and the input string itself. The structure of the resulting match array itself does not change (it still would have own index, input, and (optional) groups properties), but rather the value of each element would merely be the result as mapped through the provided mapping function.

In addition, we propose a similar change to both String.prototype.match and String.prototype.matchAll.

Prior Art


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

// offsets are relative to start of the match:
const s1 = "xaaaz";
const m1 = re1.exec(s1, (start, end) => [start, end]);
m1[0][0] === 1;
m1[0][1] === 5;
s1.slice(...m1[0]) === "aaaz";

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

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

// capture groups that are not matched (either optional or in the unmatched alternative of a
// disjunction) have an offset of -1:
const m2 = re1.exec("xaaay", (start, end) => start === -1 ? null : [start, end]);
m2[1] === null;
m2.groups["Z"] === null;

// the following two statements are functionally equivalent:
re1.exec(text, (start, end, input) => input.slice(start, end));


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.
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