Regular Expression for validating DNS label ( host name)

Question

I would like to validate a hostname using only regualr expression.

Host Names (or \'labels\' in DNS jargon) were traditionally defined by RFC 952 and RFC 1123 and m

Answer 1

While the accepted answer is correct, RFC2181 also states under Section 11, "Name Syntax":

The DNS itself places only one restriction on the particular labels that can be used to identify resource records. That one restriction relates to the length of the label and the full name. [...] Implementations of the DNS protocols must not place any restrictions on the labels that can be used. In particular, DNS servers must not refuse to serve a zone because it contains labels that might not be acceptable to some DNS client programs.

This in turn means other characters such as underscores should be allowed.