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<script type="text/javascript">
function verificaNumero(event) {
    var keyCode = ('which' in event) ? event.which : event.keyCode;

    isNumeric = (keyCode >= 48 /* KeyboardEvent.DOM_VK_0 */ && keyCode <= 57 /* KeyboardEvent.DOM_VK_9 */) ||
        (keyCode >= 96 /* KeyboardEvent.DOM_VK_NUMPAD0 */ && keyCode <= 105 /* KeyboardEvent.DOM_VK_NUMPAD9 */) ||
        (keyCode == 0) ||
        (keyCode == 8);
    return isNumeric;
};
</script>

<input type="text" onkeydown="return verificaNumero(event);">