/*
SOURCE: http://www.mredkj.com/tutorials/validate2.html

What it does
Character-by-character validation

The point of validating each character as it's entered is to prevent unwanted characters from showing up in the first place (as opposed to showing up then being removed). In this example, character validation is accomplished by using function blockNonNumbers with the input's onkeypress event, which will return either true or false. A return value of false means the last character entered will not show up.
Entire value validation

Sometimes a single character isn't enough to determine if the entire value follows a certain format. The example on this page uses function extractNumber in the onkeyup and onblur events.

Event onkeyup is called after the character has been entered. This means that in some situations (like verifying the number of decimal places), you may see the character appear, then disappear.

Event onblur is called when focus leaves the text box. This will catch attempts at copy/pasting.

These are the events that were chosen for this example, but sometimes it will suffice to only have one event fired. It depends on the needs of the formatting. 

Example usage
<input type="text" onblur="extractNumber(this,2,true);" onkeyup="extractNumber(this,2,true);" onkeypress="return blockNonNumbers(this, event, true, true);" />

blockNonNumbers takes the following arguments
function blockNonNumbers(obj, e, allowDecimal, allowNegative)

extractNumber takes the following arguments
function extractNumber(obj, decimalPlaces, allowNegative)

More examples
Negative / 3 decimal places
onblur="extractNumber(this,3,true);" onkeyup="extractNumber(this,3,true);" onkeypress="return blockNonNumbers(this, event, true, true);" 

No negative / 2 decimal places
onblur="extractNumber(this,2,false);" onkeyup="extractNumber(this,2,false);" onkeypress="return blockNonNumbers(this, event, true, false);" 

Negative / No decimal places
onblur="extractNumber(this,0,true);" onkeyup="extractNumber(this,0,true);" onkeypress="return blockNonNumbers(this, event, false, true);" 

No negative / No decimal places
onblur="extractNumber(this,0,false);" onkeyup="extractNumber(this,0,false);" onkeypress="return blockNonNumbers(this, event, false, false);" 

No negative / Unlimited decimal places
onblur="extractNumber(this,-1,false);" onkeyup="extractNumber(this,-1,false);" onkeypress="return blockNonNumbers(this, event, true, false);" 

*/
function extractNumber(obj, decimalPlaces, allowNegative)
{
	var temp = obj.value;
	
	// avoid changing things if already formatted correctly
	var reg0Str = '[0-9]*';
	if (decimalPlaces > 0) {
		reg0Str += '\\.?[0-9]{0,' + decimalPlaces + '}';
	} else if (decimalPlaces < 0) {
		reg0Str += '\\.?[0-9]*';
	}
	reg0Str = allowNegative ? '^-?' + reg0Str : '^' + reg0Str;
	reg0Str = reg0Str + '$';
	var reg0 = new RegExp(reg0Str);
	if (reg0.test(temp)) return true;

	// first replace all non numbers
	var reg1Str = '[^0-9' + (decimalPlaces != 0 ? '.' : '') + (allowNegative ? '-' : '') + ']';
	var reg1 = new RegExp(reg1Str, 'g');
	temp = temp.replace(reg1, '');

	if (allowNegative) {
		// replace extra negative
		var hasNegative = temp.length > 0 && temp.charAt(0) == '-';
		var reg2 = /-/g;
		temp = temp.replace(reg2, '');
		if (hasNegative) temp = '-' + temp;
	}
	
	if (decimalPlaces != 0) {
		var reg3 = /\./g;
		var reg3Array = reg3.exec(temp);
		if (reg3Array != null) {
			// keep only first occurrence of .
			//  and the number of places specified by decimalPlaces or the entire string if decimalPlaces < 0
			var reg3Right = temp.substring(reg3Array.index + reg3Array[0].length);
			reg3Right = reg3Right.replace(reg3, '');
			reg3Right = decimalPlaces > 0 ? reg3Right.substring(0, decimalPlaces) : reg3Right;
			temp = temp.substring(0,reg3Array.index) + '.' + reg3Right;
		}
	}
	
	obj.value = temp;
}
function blockNonNumbers(obj, e, allowDecimal, allowNegative)
{
	var key;
	var isCtrl = false;
	var keychar;
	var reg;
		
	if(window.event) {
		key = e.keyCode;
		isCtrl = window.event.ctrlKey
	}
	else if(e.which) {
		key = e.which;
		isCtrl = e.ctrlKey;
	}
	
	if (isNaN(key)) return true;
	
	keychar = String.fromCharCode(key);
	
	// check for backspace or delete, or if Ctrl was pressed
	if (key == 8 || isCtrl)
	{
		return true;
	}

	reg = /\d/;
	var isFirstN = allowNegative ? keychar == '-' && obj.value.indexOf('-') == -1 : false;
	var isFirstD = allowDecimal ? keychar == '.' && obj.value.indexOf('.') == -1 : false;
	
	return isFirstN || isFirstD || reg.test(keychar);
}