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
– Pointy, Commented May 27, 2018 at 13:23
\1 will match exactly the same text, not the variables. If you have 12121212 it will probably match it — Yassin Hajaj
– Yassin Hajaj, Commented May 27, 2018 at 13:25
Note that ^(\d{2}-\d{2}-\d{2}-\d{2}|\d{8})$ works just fine — Yassin Hajaj
– Yassin Hajaj, Commented May 27, 2018 at 13:33
Things like (?n) / \g<n> (subroutines) are not supported in JS RegExp. See this JSFiddle to get a hint on how to get what you need. — Wiktor Stribiżew
– Wiktor Stribiżew, Commented May 27, 2018 at 14:21

Hegel F. · Accepted Answer · 2018-05-27 13:54:57Z

1

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

answered May 27, 2018 at 13:54

Hegel F.

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Wiktor Stribiżew · Accepted Answer · 2018-05-27 14:59:49Z

1

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));
}

answered May 27, 2018 at 14:59

Wiktor Stribiżew

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edmond · Accepted Answer · 2018-05-27 13:57:21Z

0

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.

answered May 27, 2018 at 13:57

edmond

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