RegExp match array offsets for ECMAScript

ECMAScript RegExp match array offsets provide additional information about the start and end position of a captured substring.

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 0 Presentation

Status

Stage: 2 Champion: Ron Buckton (@rbuckton)

For detailed status of this proposal see TODO, below.

Authors

• Ron Buckton (@rbuckton)

Motivations

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.

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

• Oniguruma NodeJS bindings: captureIndices property
• .NET: Capture.Index Property
• Java: Matcher.start(int) Method

Examples

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;

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.
• The ECMAScript editor has signed off on the current spec text.

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.