How Do I Access Captured Subgroups in JavaScript Regular Expressions?

Accessing Matched Groups in JavaScript Regular Expressions

When matching a string against a regular expression, it's crucial to understand how to retrieve the captured subgroups. In JavaScript, this is achieved using capturing groups within the regular expression.

Consider the following example:

var myString = "something format_abc"; // Target substring: "abc"

var arr = /(?:^|\s)format_(.*?)(?:\s|$)/.exec(myString);

console.log(arr); // [" format_abc", "abc"]
console.log(arr[1]); // undefined
console.log(arr[0]); // format_undefined

Reason for Undefined Output:

While the regular expression correctly matches the substring "abc," accessing arr[1] returns undefined due to incorrect printing in the console.log statements. console.log interprets the string %A as a formatting placeholder and attempts to substitute it with the second parameter, which is undefined in this case.

Accessing Matched Groups:

To extract the matched group, use arr[1]. This will correctly return "abc."

Iterating Over Multiple Matches (ES2020):

JavaScript 2020 introduced the matchAll method, which yields an iterator that can be used to iterate over all matches in a string.

const string = "something format_abc";
const regexp = /(?:^|\s)format_(.*?)(?:\s|$)/g;
const matches = string.matchAll(regexp);

for (const match of matches) {
  console.log(match);
  console.log(match.index);
}

This approach provides a cleaner and more intuitive way to handle multiple matches.

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