Всем спасибо
<input type='text' value='' id='text1'><input type='button' value='push' onclick='delet1()'>
<script>
function delet1() {
var str = document.getElementById('text1').value
var text1 = parseInt(str,10)
if( /[0-9]/.test(text1) == true ) {
document.getElementById('text1').value = ''
}
else {
alert('false')
}
}
</script>