esempio... da adattare alle tue esigenze

codice:
<SCRIPT LANGUAGE="JavaScript">
<!--
function ctr(f) {
v = f.imp.value;
RE = new RegExp(/^\d+[\.|\,]\d{1,2}$/)
alert(RE.test(v))
}
//-->
</SCRIPT>
<FORM>
<INPUT TYPE="text" NAME="imp">
<INPUT TYPE="button" onclick="ctr(this.form)">
</FORM>
ciao