My answer to the question "https://stackoverflow.com/questions/6449367/c-sharp-email-address-validation/6459786" might help you: https://stackoverflow.com/a/6459786/467473
It implements an email address validator in C#. You don't care about the "local part" (the bit left of the @ in email addresses, or the @ itself. The bit that's interesting to you is the fqdn production and that should be fairly straightforward to map to a Javascript regular expression.
On further consideration, it does sound like what you are describing is a DNS label, a single segment of a DNS name.
If what you want is to validate an RFC-compliant DNS label, then this regula expression ought to do you:
const rxDnsLabel = /^[A-Z]([A-Z0-9-]{0,61}[A-Z0-9])?$/i;
Breaking it down:
^ — anchors the start of the match to start-of-text, followed by[A-Z] — a US-ASCII letter, followed by( — an optional group, consisting of
[A-Z0-9-] — a US-ASCII letter, decimal digit or hyphen (-)
{0,61} — repeated 0 to 61 times, followed by
[A-Z0-9] a single US-ASCII letter or decimal digit
)? — the whole of which is optional$ — anchors the end of the match to end-of-text
If you need to match a DNS name consisting of multiple labels, it's not that much more complicated. Just need to allow the optional extra segments:
const rxDnsLabel = /^([A-Z]([A-Z0-9-]{0,61}[A-Z0-9])?)([.][A-Z]([A-Z0-9-]{0,61}[A-Z0-9])?)*$/i;
The only difference here is that the initial label is allowed to be followed by zero or more additional labels, each separated with a period/full stop ( .).
Edited to note: If you require support for internationalized (punycode) domain names, I can't guarantee that this will match them
mostly because I never needed to do so, and so haven't tested this against them. For details on internationalized (punycode) domain names, see the relevant RFs:
