Get index of each capture in a JavaScript regex

capturing-group, javascript, regex

Solution

There is currently a proposal (stage 4) to implement this in native Javascript:

RegExp Match Indices for ECMAScript

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

...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.

Here's an example of how things would work. The following snippets run without errors in, at least, Chrome:

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

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

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

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

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

So, for the code in the question, we could do:

const re = /(a).(b)(c.)d/d;
const str = 'aabccde';
const result = re.exec(str);
// indices[0], like result[0], describes the indices of the full match
const matchStart = result.indices[0][0];
result.forEach((matchedStr, i) => {
  const [startIndex, endIndex] = result.indices[i];
  console.log(`${matchedStr} from index ${startIndex} to ${endIndex} in the original string`);
  console.log(`From index ${startIndex - matchStart} to ${endIndex - matchStart} relative to the match start\n-----`);
});

Output:

aabccd from index 0 to 6 in the original string
From index 0 to 6 relative to the match start
-----
a from index 0 to 1 in the original string
From index 0 to 1 relative to the match start
-----
b from index 2 to 3 in the original string
From index 2 to 3 relative to the match start
-----
cc from index 3 to 5 in the original string
From index 3 to 5 relative to the match start

Keep in mind that the `indices` array contains the indices of the matched groups relative to the start of the string, not relative to the start of the match.

A polyfill is available here.

Problem

I want to match a regex like `/(a).(b)(c.)d/` with `"aabccde"`, and get the following information back: ``` "a" at index = 0 "b" at index = 2 "cc" at index = 3 ``` How can I do this? String.match returns list of matches and index of the start of the complete match, not index of every capture. Edit: A test case which wouldn't work with plain indexOf ``` regex: /(a).(.)/ string: "aaa" expected result: "a" at 0, "a" at 2 ``` Note: The question is similar to Javascript Regex: How to find index of each subexpression?, but I cannot modify the regex to make every subexpression a capturing group.

Original source