[рони]

MC-XOBAHCK,

<input type="text" id="test">
<script>
    document.querySelector('#test').addEventListener('input', function() {
        this.value = this.value.replace(/,/, '.').replace(/\./g, function(a, b, c) { 
    return c.indexOf(a) == b ? a : ''
}).replace(/[^\d.]/, '');
    });
</script>