sub verify_FQDN {
  return $_[0] =~ m{
    \A
    (?:
      (?! \d+ \. )
      [a-zA-Z0-9]+ (?: -+ [a-zA-Z0-9]+ )*
      \.
    ) +
    [a-zA-Z]{2,4}
    \z
  }x;
}