Very quick function to test a domain name is valid:

    function validDomain($domain)
        // Test domain is actually valid
        if (preg_match('/^[a-z]+([a-z0-9-]*[a-z0-9]+)?(.([a-z]+([a-z0-9-]*[a-z0-9]+)?)+)*$/i', $domain))
            return TRUE;
        return FALSE;

The key here is the regular expression (delimited by the forward slashes at each end):


This basically translates as follows

  • ^[a-z]+    : First chacter must be alphabet
  • ([a-z0-9-]*[a-z0-9]+)    : A group. The first character may be in the range, a-z or 0-9, or a hyphen. This pattern can repeat, although the last character may not be a hyphen
  • ?    :  That last group is optional
  • (.     :  Start of next group, and it must be a dot
  • ([a-z]+ : First letter must be in range a-z
  • ([a-z0-9-]*[a-z0-9]+)?  :  Then an optional group of a-z, 0-9 and hypen, though last  character may not be hyphem (as above)
  • )+)  : Do the last bit at least once
  • :  The last bit is optional and can repeat as much as you like
  • $:   And that’s the end of the string

It’s not a perfect reg-exp for the job, but it does the job where a quick quick is required (user signups fro example), rather than mission critical domain testing.

A good tool for testing your regular expressions can be found here