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Form validation for "+" and "-"

Question:

I have a form where the user has to insert either a "+" sign, a "-" sign, or can leave the field blank.

The problem is I'm finding it difficult to validate the input. I've tried using "\+" for the + sign (as it's a metacharacter) but I can't seem to get it to work.

code at the moment is very simple

I have the variables from the form and then just

Code:

if (sign.value != "\+" || sign.value != "-" || sign.value != "")
{
	alert("Your value is " + sign.value + "You can only enter a +, - or leave it blank");
        return false;
}
else return true;

where sign is the name of the field. Instead of printing the value of the + or -, the alert box is printing NaN.

Sorry if this has been answered before, I had a look and searched all over but couldn't find anything about it.

Thanks in advance for the help

Answer:

This may not be what your looking for but have you thought of using a drop-down with only those three options and therefore no validation would be required?

otherwise you could use something like this:

Code:

 
<HTML>
<HEAD>
<SCRIPT>
//* Forced +-_b - allows whitespace *//
function ForcedplusMinusBlank(o)
{
if((/[^\+\- ]/i).test(o.value))
{
	o.value=o.value.replace(/([^\+\- ])/g,"");
	if (navigator.appName != "Microsoft Internet Explorer")
	{
alert('Unacceptable characters have been removed from last entry!');
return false;
	} 
}
}
</SCRIPT>
</HEAD>
<BODY>
<INPUT onchange="ForcedplusMinusBlank(this)" 
onattrmodified="ForcedplusMinusBlank(this)" 
onpropertychange="ForcedplusMinusBlank(this)" 
type="text" id="InputBoxName" 
name="InputBoxName" size="1">
</BODY>
</HTML>

Bascially, this will work for IE and Netscape it disallows anything but entries + - or blank (whitespace).
In IE you cant even enter the values in Netscape it remoes them after you have left the field and warns you.

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