A complex domain name match rewrite rule

Question

I am trying to create a regex rule for the following,

The rule is for matching domain names with the following conditions

1. need to mach domain name , foo.com
2. need to match a.com
3. need to match domain name start with underscrore and followed by a charector number ( _f.a.com, _foo.foo.com , don't match _.com or _*.a.com)
4. need to match *.a.com or *.foo.com , don't match *foo.com or *_foo.com
5. don't end with dot

So far I reached the following regex

^(([a-zA-Z0-9]|\*\.[a-zA-Z0-9])|[a-zA-Z0-9_][a-zA-Z0-9-]){0,254}[a-zA-Z0-9](?:\.[a-zA-Z0-9]{2,})+$

The problem is it doesn't match *.a.com , it will match all other rule

Any help is appreciated

Answer 1

• The pattern does not match _f.a.com because the _ and the f can be matched here [a-zA-Z0-9_][a-zA-Z0-9-], but the following character is a dot that can not be matched by any of the alternatives and also not by the [a-zA-Z0-9] part right after the alternation
• The pattern does not match *.a.com for almost the same reason, only this time for *.

You can match all the examples using:

^(?:_|\*\.)?[a-zA-Z0-9]+(?:\.[a-zA-Z0-9]+)*$

Explanation

• ^ Start of string
• (?:_|\*\.)? Optionally match either _ or .*
• [a-zA-Z0-9]+ Match 1+ times any of the ranges
• (?:\.[a-zA-Z0-9]+)* Optionally repeat the previous with a leading dot
• $ End of string

See a regex demo.

You can make the pattern more specific if you want, for example if there should be 2 or more characters after the last dot:

^(?:_|\*\.)?[a-zA-Z0-9]+(?:\.[a-zA-Z0-9]+)*\.[a-zA-Z0-9]{2,}$

You can use the quantifier {0,254} for any of the character classes.