function checkEmailFormat ( email )
{
   var len = email.length;
   var ck =0;
   if(len==0)
      return(-1);
   for(var i=0;i<len;i++)
   {  var c= email.charAt(i);
      if(!((c>="A"&&c<="Z")||(c>="a"&&c<="z")||(c>="0"&&c<="9")||(c=="-")||(c=="_")||(c==".")||(c=="@")))
         return (-3);
   }
   if((email.indexOf("@")==-1)||(email.indexOf("@")==0)||(email.indexOf("@")==(len-1)))
      return(-4);
   if((email.indexOf("@")!=-1)&&(email.substring(email.indexOf("@")+1,len).indexOf("@")!=-1))
      return(-4);
   if((email.indexOf(".")==-1)||(email.indexOf(".")==0)||(email.lastIndexOf(".")==(len-1)))
      return(-5);
   return (0);
}


function check_mail( email )
{  if (email!=''){
   var ck= checkEmailFormat(email);

   if(ck==-1)
   {
      alert("You have to provide your E-mail address.\n");
      return false;
   }
   else if(ck==-3)
   {
      alert("The E-mail address should \n contain only English characters, \n '-', or '_'. Other characters cannot be used.\n");
      return false;
   }
   else if (ck==-4)
   {
      alert("Your E-mail address is invalid.\n");
      return false;
   }
   else if (ck==-5)
   {
      alert("Your E-mail address is incomplete.\n");
      return false;
   }}
   return true;
}