Javascript regex to match fully qualified domain name, without protocol, optional subdomain

Question

I haven't been able to find this one and what I'm trying isn't quite working out for me.

I want to match only domains that:

don't contain a protocol (http, https, ftp)
optionally include a subdomain
don't start with a hyphen but can contain a hyphen

Example domains that would match:

domain.com
example.domain.com
example.domain-hyphen.com
www.domain.com
example.museum

Example domains that would not match:

http://example.com
subdomain.-example.com
example.com/parameter
example.com?anything
www.subdomain.domain.com

What I've currently got:

/^(?!:\/\/)(^[a-zA-Z0-9])?.[a-zA-Z0-9-]+\.[a-zA-Z]{2,6}?$/i

It's not matching protocols, allowing hyphen inside the domain, not allowing trailing characters after the TLD, and is allowing a subdomain (but only 1 character).

I still need to allow subdomains of any length, not allow www.subdomain.domain.com and not allow a leading hyphen.

How do you define "domain" then? Every word with at least one dot in it? — Bergi, May 09 '13 at 14:13
Not sure I understand. `domain` precedes the TLD and can only contain letters, numbers and a hyphen. — Jared Eitnier, May 09 '13 at 14:15
Well, that was my (failed) attempt at allowing subdomains. Basically that's where I got stuck and the last thing I added to this regex. — Jared Eitnier, May 09 '13 at 14:17

score 15 · Accepted Answer · answered May 09 '13 at 14:17

15

Try

/^(?!:\/\/)([a-zA-Z0-9]+\.)?[a-zA-Z0-9][a-zA-Z0-9-]+\.[a-zA-Z]{2,6}?$/i

answered May 09 '13 at 14:17

dda

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I know this is old, but it should be updated to allow domains such as .photography, which has more than 6 characters – Nico Aug 25 '14 at 15:34
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@dda I see that you have deleted your TSE profile (I came to know after losing 310 points). Although I don't have any right to ask, I hope everything is well. That forum is boring without your edits. – Hanky Panky Sep 18 '18 at 08:37
@HankyPanky One of my answers was edited, against, my wish, and locked, to prevent to change it back. It was the last straw of a series of things that made me regret joining TSE in the first place. It became obvious my work wasn't appreciated by the moderators... – dda Sep 19 '18 at 12:18
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I can't say anything on behalf of the moderators then but I have always found your contribution valuable. I used to think you'd make a great moderator one day (I still think so). I'm disappointed to see you go and I hope you guys sort it out and be back. – Hanky Panky Sep 19 '18 at 19:23
@HankyPanky Also, right after I left, there was an attempt at downvoting me 27 times in a row on Expats (where I am no longer active either). This was quickly overturned, but it obviously was a TSE member who was unhappy with the points loss. I also on chat that the person who censored my answer was unhappy about my leaving. Should I thought about his actions first. But I also saw that not everybody was unhappy... So, farewell. – dda Sep 25 '18 at 07:07
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I don’t know why TSE has become like that lately but then it’s such a big crowd that I can’t ever speak for someone. I will definitely miss your contribution and I hope you do come back. A few people not doing the right thing don’t really represent the whole community. – Hanky Panky Sep 25 '18 at 18:25
doesn't work with new domains, even something as simple as inte.io failed – Neo Nov 27 '22 at 13:02

score 4 · Answer 2 · answered May 09 '13 at 14:24

4

Let's analyse your regex:

^(?!:\/\/)

This is pretty useless. While it indicates the intention of the regex, it's unnecessary since the following characters are not allowed to contain slashes anyway.

(^[a-zA-Z0-9])?.

I think you wanted this to be ^([a-zA-Z0-9]+\.)?. Your dot is not escaped, and would be preceded by only one optional character at the string beginning.

[a-zA-Z0-9-]+

If you want this not to begin with a hyphen, you either could use a negative lookahead or better just [a-zA-Z0-9][a-zA-Z0-9-]*.

\.[a-zA-Z]{2,6}?

Not sure what the question mark does here. There's no backtracking anyway?

/i

This renders the explicit [a-zA-Z] useless, one would be enough. Or omit the i flag.

All these things together, we will end up with

/^([a-z0-9]+\.)?[a-z0-9][a-z0-9-]*\.[a-z]{2,6}$/i

answered May 09 '13 at 14:24

Bergi

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I know this is old, but it should be updated to allow domains such as .photography, which has more than 6 characters – Nico Aug 25 '14 at 15:35
Sure, however I've only analyzed the regex that was presented in the question and got it working (without commenting on the meaning) – Bergi Aug 25 '14 at 15:41
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Yes, you're right, sorry... But when I saw it I thought if people hunt for the answer they might copy this one without even considering its limitations... So leave it as is – Nico Aug 26 '14 at 11:39

score 4 · Answer 3 · answered May 23 '17 at 06:52

4

Try this:

^[a-zA-Z0-9][a-zA-Z0-9-]{1,61}[a-zA-Z0-9](?:\.[a-zA-Z]{2,})+$

Demo

answered May 23 '17 at 06:52

Dharmesh Hadiyal

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Just what I was looking for. Thanks! – Bryan McGrane Sep 17 '19 at 23:02

score 1 · Answer 4 · edited Sep 20 '18 at 04:47

The RegEx I came up with while doing the Hostname/FQDN validation in Javascript:

FQDN:

^(?!:\/\/)(?!.{256,})(([a-z0-9][a-z0-9_-]*?\.)+?[a-z]{2,6}?)$/i

Hostname or FQDN

^(?!:\/\/)(?!.{256,})(([a-z0-9][a-z0-9_-]*?)|([a-z0-9][a-z0-9_-]*?\.)+?[a-z]{2,6}?)$/i

Both expressions use lookahead to check the total string length, which can be up to 255 characters. They also do a lazy check .{x,y}?.

Note that it is using case insensitive match /i.

Javascript regex to match fully qualified domain name, without protocol, optional subdomain

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