There have been a number of pages around T'internet on regexps for determining an IP number, and I needed a slight variation.... which has led me to a really natty regexp for the base use-case:

chomp $ip;
my $quad = '([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])';
if ($string =~ /^(${quad}\.){3}${quad}$/) {
  ## have valid IP
}
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All previous examples have been long strings defining each quad separately... I use two simple tricks:

1. use variable substitution for the actual pattern, and
2. use the repeat operator to define three repeats of the first section

oh... and ${quad} is just a way of ensuring that the right variable name is used... a-la $foo = "${bar}barella" looks for $bar and appends barella to it

In my case, I wanted to try to guess between a (partial) IP number, a hostname, or some other string (a name) - so I have:

  # $q contains the string to test
  my $quad = '([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])';
  given ($q) {
    when (    # Is it an IP?
      # matches 127 or 127. or 127.0 or 127.0. or...
      # does not match any quad above 255, or 127.0.0.1.
      #   as the fifth dot fails the match
      /^((${quad}\.){0,2}${quad}\.?|(${quad}\.){3}${quad})$/
    ) {
      $type = 'ip';
    } ## end when ( ...)
    when (/\w+\.\w+/) {    # Is it a domain name?
      # words.words
      $type = 'url'
    }
    default { $type = 'name' }    # it must be a name then...
  } ## end given
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