<SCRIPT LANGUAGE="JavaScript">
<!-- Original: Mark Henwood (mark_henwood@hotmail.com) -->
<!-- Begin
function commaSplit(srcNumber) {
var txtNumber = '' + srcNumber;
if (isNaN(txtNumber) || txtNumber == "") {
alert("Ошибка! Вы ввели некорректное число.");
fieldName.select();
fieldName.focus();
}
else {
var rxSplit = new RegExp('([0-9])([0-9][0-9][0-9][,.])');
var arrNumber = txtNumber.split('.');
arrNumber[0] += '.';
do {
arrNumber[0] = arrNumber[0].replace(rxSplit, '$1,$2');
} while (rxSplit.test(arrNumber[0]));
if (arrNumber.length > 1) {
return arrNumber.join('');
}
else {
return arrNumber[0].split('.')[0];
  }
  }
}
// End -->
</script>

<form name="commas">
Введите число:  
<input type=text name="inpNumber" size=20 value="">
<input type=button value="ОК" onClick="document.commas.inpNumber.value=commaSplit(document.commas.inpNumber.value);">
</form>