<input> <script> document.body.children[0].onkeyup = function () { var reg = /[а-яА-ЯёЁ]/g; if (this.value.search(reg) != -1) { this.value = this.value.replace(reg, ''); } } </script>
<input id="inp"> <script> document.getElementById('inp').onkeyup = function () { var reg = /[а-яА-ЯёЁ]/g; if (this.value.search(reg) != -1) { this.value = this.value.replace(reg, ''); } } </script>