用javascript限制一个文本框只能输入数字

<input name=mlizhong size=4 value='' maxlength=4 
  onkeypress="return event.keyCode>=48&&event.keyCode<=57" 
  onpaste="var s=clipboardData.getData('text'); if(!/\D/.test(s)) value=s.replace(/^0*/,''); return false;" ondragenter="return false" 
  style="ime-mode:disabled"  onkeyup="if(/(^0+)/.test(value))value=value.replace(/^0*/, '')">