Regex for domain verification with Javascript

Question

I want to create a regular expression to verify domain names.

I've searched a lot on StackOverflow and I found this solution, which ignores the http and folders and checks only for domain.

QUOTE

var pattern = new RegExp('^[a-zA-Z0-9][a-zA-Z0-9-]{1,61}[a-zA-Z0-9](?:\.[a-zA-Z]{2,})+$', 'i');

But since I'm not a wizard and I've long ago given up on trying to understand Regex, I placed it on Regex101 which shows me that 'localhost' is not a match. The problem is when I execute the following code (jsFiddle here):

var origin = 'localhost';
var pattern = new RegExp('^[a-zA-Z0-9][a-zA-Z0-9-]{1,61}[a-zA-Z0-9](?:\.[a-zA-Z]{2,})+$', 'i');
console.log(origin, pattern.test(origin));

It returns true (localhost should not be matched). Why?

Edit: For some reason, people thought I WANTED localhost to be recognized as a domain. BUT IT'S THE OPPOSITE! If you read the question, you'll see that it's returning true although it should return false.

For a start, if you're using a string to define the regex, you need to escape that backslash. — T.J. Crowder
– T.J. Crowder, Commented Feb 26, 2015 at 18:08
Try: pattern = /^[a-zA-Z0-9][a-zA-Z0-9-]{1,61}[a-zA-Z0-9](?:\.[a-zA-Z]{2,})+$/; — anubhava
– anubhava, Commented Feb 26, 2015 at 18:09
@T.J.Crowder I thought that the backslash was escaping the dot... — user3310334
– user3310334, Commented Feb 26, 2015 at 18:09
More importantly: This regex completely fails to handle anything other than the letters A-Z in the English alphabet. We have more expressive TLDs now. I don't think I'd try to validate anything other than that I had at least one character, a dot, and at least one character. And that will, of course, fail for localhost as well, because localhost isn't a domain name. — T.J. Crowder
– T.J. Crowder, Commented Feb 26, 2015 at 18:10
@minseong: It would do in an regex literal. In a string literal, \. is . (because the backslash is an escape in the string literal that escapes the dot, which doesn't need escaping, and ends up doing thing; the regex engine never sees that backslash). — T.J. Crowder
– T.J. Crowder, Commented Feb 26, 2015 at 18:10

score 3 · Accepted Answer · 2024-02-15 05:06:11Z

3

In your Regex101, you are entering a literal regex, not a string. As your regex is correct (it is) localhost doesn't match. However, in your code, you are creating your regex using strings (bad idea), which means you have to escape each backslash.

You can prevent the issue using Regexp literals.

Just use your original regex with forward-slashes, i.e.

var pattern = /^[a-zA-Z0-9][a-zA-Z0-9-]{1,61}[a-zA-Z0-9](?:\.[a-zA-Z]{2,})+$/i;

Or just escape backslashes:

var pattern = new RegExp('^[a-zA-Z0-9][a-zA-Z0-9-]{1,61}[a-zA-Z0-9](?:\\.[a-zA-Z]{2,})+$', 'i');

edited Feb 15, 2024 at 5:06

answered Feb 26, 2015 at 18:12

user3310334

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6 Comments

Marco Aurélio Deleu Over a year ago

I don't want to, I want it to fail. This answer helps me.

zb' Over a year ago

antitest 1.c.test.com

zb' Over a year ago

(/^[a-zA-Z0-9][a-zA-Z0-9-]{1,61}[a-zA-Z0-9](?:\.[a-zA-Z]{2,})+$/i).test('1.c.test.com') false while it is valid domain name

zb' Over a year ago

or more real one: www.w3.org

user3310334 Over a year ago

@eicto I didn't make the regex. I just had to show OP why his code didn't work as they'd expected.

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