Field Validation As Float With Certain Integer And Certain Decimal

i have to do in the onkeypress or in onchange the float (real) field validation I try something:

function ValidaCampo(nomeCampo,TotInteri,TotDecimali)
{
DecimalPos=NomeCampo.value.Indexof(',');
Lungh=TotInteri+TotDecimali+1; //sum the ineger + decimal +
decimal separator

if ((window.event.charcode==',') && (DecimalPos>-1))
//i think exist a decimal separator so i don't add any other
else
if (DecimalPos<>TotInteri+1)
//i haven' t insert too much integer
else
if i haven't add any decimal, i add a separator with two zero value
else
if nomecampo.length<>Lungh
//error

}