Preventing a keystroke value from being added to string.

[Claus Mygind]

with the "onKeyPress " event handler I see that I can capture and examine the value of a keystroke before it is displayed on the screen. I have made good use of this for single character fields where I want to examine if the value is correct and also convert the value to upper case like so:

  
<input 
   type="text" 
   name="f1" 
   id  ="f1"  
   maxLength="1"
   onKeyPress="return isHorS(event);" 
 />

function isHorS(evt)
{
   evt = (evt) ? evt : event; 
   var charCode = (evt.charCode) ? evt.charCode : ((evt.keyCode) ? evt.keyCode : ((evt.which) ? evt.which : 0));   

      if (charCode != 8 && charCode != 9 && charCode != 32 && charCode != 72 && charCode != 104 && charCode != 83 && charCode != 115 )
      {  
            alert("You must either leave blank or enter H or S!"); 
 			document.getElementById("f1").value = "";
      }else{
	      if (charCode == 104  || charCode == 115 )
		  {
			var cUp = String.fromCharCode(charCode)
			document.getElementById("f1").value = cUp.toUpperCase();
		  }
	  }

}

As I said this works great when I am only dealing with one character. But for longer strings than one character, I would like to prevent unwanted characters from becoming part of the string. Can I use this technique by intercepting what the user is typing give them an alert and then cancel that keystroke entry?

[Logician]

It's much simpler to use a regular expression to remove unwanted characters from the string after each keystroke. Use the onkeyup event.

[Claus Mygind]

great do you have an example I can use. although I did get it to work like this:

 onKeyPress=" return isValidCode(event, this);"

function isValidCode(evt, obj)
{
   evt = (evt) ? evt : event; 
   var charCode = (evt.charCode) ? evt.charCode : ((evt.keyCode) ? evt.keyCode :  
       ((evt.which) ? evt.which : 0));   
	if ( charCode == 8 || 
		 charCode == 9 || 
		(charCode > 47 && charCode < 58) || 
		(charCode > 64 && charCode < 91) ||	
		(charCode > 96 && charCode < 123) )
	{
		if (charCode > 96 && charCode < 123)
		{
			Here I would like to make it an uppercase value
		}
	}else{
		alert("That is not a valid character ");
		return false;
	}
}

Perhaps your example of a regular expression can also convert the keystroke upper case if allowed?

[Claus Mygind]

Ok I have learned that only IE let's you intercept a keystroke and alter it before it reaches the form (generally considered a security risk as programmer can alter what the user is entering)

var charCode = event.keyCode;

event.keyCode = charCode - 32

And as I am working exclusively with fireFox that will not work for me.

will look at the keyup option