var IP_Validator_Regexp='(1\d{0,2}|2(?:[0-4]\d{0,1}|[6789]|5[0-5]?)?|' + 
 '[3-9]\d?|0)\.(1\d{0,2}|2(?:[0-4]\d{0,1}|[6789]|5[0-5]?)?|[3-9]\d?|0)' +
 '\.(1\d{0,2}|2(?:[0-4]\d{0,1}|[6789]|5[0-5]?)?|[3-9]\d?|0)\.(1\d{0,2}' + 
 '|2(?:[0-4]\d{0,1}|[6789]|5[0-5]?)?|[3-9]\d?|0)(\/(?:[012]\d?|3[012]?' +
 ')){0,1}$';
 
   // how to use it in Javascript
 
   var regex =/IP_Validator_Regexp/;
   if (!(regex.test(document.getElementById('user_ip').value))) {
     alert('This is not a valid IP number');
     return;
   }