Reuse capturing group pattern in JS regex

Question

I want to test whether a string is 8 digits or the same number can be divided into 4 parts separated by a -.

I have a regex that does work:

/^\d{2}([-]?)(\d{2})\1\d{2}\1\d{2}$/

And I can prove:

/^\d{2}([-]?)(\d{2})\1\d{2}\1\d{2}$/.test('12345678'); // true
/^\d{2}([-]?)(\d{2})\1\d{2}\1\d{2}$/.test('12-34-56-78');  // true
/^\d{2}([-]?)(\d{2})\1\d{2}\1\d{2}$/.test('12-45-7810'); // false
/^\d{2}([-]?)(\d{2})\1\d{2}\1\d{2}$/.text('-12-45-78-10'); //false

I would have liked to have created a group from \d{2} but I tried this:

/^(\d{2})([-]?)\1\2\1\2\1$/

But it does not match anything.

It *does* match something: try "22-22-22-22". By using `\1` it only matches the exact same two digits. — Pointy, May 27 '18 at 13:23
`\1` will match exactly the same text, not the variables. If you have `12121212` it will probably match it — Yassin Hajaj, May 27 '18 at 13:25
Note that `^(\d{2}-\d{2}-\d{2}-\d{2}|\d{8})$` works just fine — Yassin Hajaj, May 27 '18 at 13:33
Things like `(?n)` / `\g` (subroutines) are not supported in JS RegExp. See [this JSFiddle](https://jsfiddle.net/0w91h0x6/) to get a hint on how to get what you need. — Wiktor Stribiżew, May 27 '18 at 14:21
Did [my solution](https://stackoverflow.com/a/50553431/3832970) help? — Wiktor Stribiżew, Nov 22 '18 at 09:33

score 1 · Answer 1 · answered May 27 '18 at 13:54

1

You can write even shorter: ^\d\d(-\d\d){3}$|^\d{8}$

answered May 27 '18 at 13:54

Hegel F.

125
6

score 1 · Answer 2 · answered May 27 '18 at 14:59

You cannot make backrefernces (references to actual values captured with group patterns) work as subroutines (constructs "repeating" some group patterns, not the actual captured values).

You can only define pattern parts and use them to build longer patterns:

const n = "\\d{2}";
const h = "-?";
const rx = new RegExp(`^${n}(?:${h}${n}){3}$`);
console.log(rx.source); // ^\d{2}(?:-?\d{2}){3}$
// ES5:
// var rx = new RegExp("^" + n + "(?:" + h + n+ "){3}$");
 
const strs = ['12345678', '12-34-56-78', '12-45-7810', '-12-45-78-10'];
for (let s of strs) {
 console.log(s, "=>", rx.test(s));
}

score 0 · Answer 3 · answered May 27 '18 at 13:57

Have you tried:

const r = new RegExp(/^-?\d{2}-?\d{2}-?\d{2}-?\d{2}$/);
console.log(r.test("12345678")); // true
console.log(r.test("12-34-56-78")); // true
console.log(r.test("12-45-7810")); // true
console.log(r.test("-12-45-78-10")); // true

This works on my end.

Reuse capturing group pattern in JS regex

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