c:\@Work\Perl\monks>perl -wMstrict -le
"my $regex = qr/(2[0-4]|1?[0-9])?[0-9]|25[0-5]/;
  while (<>) {
      chomp;
      if ($_ =~ /^$regex$/) {
          print qq{'$_' matched};
      } else {
          print qq{'$_' did not match};
      }
  }
"
0
'0' matched
1
'1' matched
100
'100' matched
1000
'1000' did not match
25
'25' matched
255
'255' matched
256
'256' did not match
a1
'a1' did not match
1a
'1a' did not match
11
'11' matched
111
'111' matched
222
'222' matched
333
'333' did not match