Java script Code Problem

Hi All,

I've wrote the following java script in order to validate empty fields, URL's and emails in my form fields although it is not working properly (not validating) so can someone please tell me what exactly I am doing wrong:

==============================
//validationsc.js
==============================

function checkURL(url){
 var expression = new  RegExp("^(((ht|f)tp(s?))\://)?(www.|[a-zA-Z].)[a-zA-Z0-9\-\.]+\.(com|edu|gov|mil|net|org|biz|info|name|museum|us|ca|uk)(\:[0-9]+)*(/($|[a-zA-Z0-9\.\,\;\?\'\\\+&%\$#\=~_\-]+))*$", "i");
 if (!expression.test(url) )
  return false;
 else
  return true;
}

function validEmail(emailstr) {
  return (emailstr.indexOf("@") > 0) && (emailstr.indexOf(".") > 2);
}

function validContent(myfrm) {
  var errempty = 'The following required field(s) are missing:\n\n';
  var erremail = 'Please provide a valid email address\nin Email field which is required.\n';
  var errlinkback = 'Please provide a valid link back address\nin the Link Back field which is required.\n';
 
  var fields = '';
  if ( myfrm.name.value == '' ) {
      fields = fields + ' Name\n';
  }
  if ( myfrm.companyurl.value == '' ) {
      fields = fields + ' Company URL\n';
  }  
  if ( ! fields == '' ) {
      alert(errempty + fields);
      return false;
  }

  if( !checkURL((myfrm.linkback.value) ){
      alert(errlinkback);
      return false;                        
  }
  if ( ! validEmail(myfrm.companyemail.value) ) {
      alert(erremail);
      return false;
  }  
  return true;
}


==============================
//form.html
==============================
...
...
<SCRIPT LANGUAGE="JavaScript" SRC="validationsc.js"> </SCRIPT>
...
...
<form name="linkexform" method="post" action="linkexform.php"  onSubmit='return validContent(this)'>
...
...
</form>



Thanks in advance for your help