RegEx js: how to correctly reuse patterns using capture groups?

Question

let string="James 18 Jenny 38 ";
let regEx=/(\w+\s\d+\s)\1/;
let result=string.match(regEx);

console.log(result)

I was trying to understand how can I reuse patterns with capture groups but the console returns null:

Can somebody explain me please why with this approach it doesn't work? If I duplicate as below it returns in the way I imagined, thus I probably miss something about the syntax, but I don't understand what.

let string="James 18 Jenny 38 ";
let regEx=/(\w+\s\d+\s)(\w+\s\d+\s)/;
let result=string.match(regEx);

console.log(result)

Answer 1

\1 doesn't reuse the pattern, it looks for another instance of what matched the pattern. In your example, it's looking for another James 18 and not finding it.

You could use a {2} quantifier to look for two matches of a pattern next to each other, but then the capture group would either include just one of them or both of them (depending on where and how you applied the quantifier). For instance, here it has both:

let string = "James 18 Jenny 38 ";
let regEx = /((?:\w+\s\d+\s){2})/;
let result = string.match(regEx);

console.log(result[1]);

Or use just one instance of the pattern in a loop:

let string = "James 18 Jenny 38 ";
let regEx = /(\w+\s\d+\s)/g;
let result;

while ((result = regEx.exec(string)) !== null) {
    console.log(result[1]);
}