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<input type="text" id="kahao">
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 $(function() {
     $('#kahao').on('keyup', function(e) {
         //只对输入数字时进行处理
         if ((e.which >= 48 && e.which <= 57) ||
             (e.which >= 96 && e.which <= 105)) {
             //获取当前光标的位置
             var caret = this.selectionStart
             //获取当前的value
             var value = this.value
             //从左边沿到坐标之间的空格数
             var sp = (value.slice(0, caret).match(/\s/g) || []).length
             //去掉所有空格
             var nospace = value.replace(/\s/g, '')
             //重新插入空格
             var curVal = this.value = nospace.replace(/(\d{4})/g, "$1 ").trim()
             //从左边沿到原坐标之间的空格数
             var curSp = (curVal.slice(0, caret).match(/\s/g) || []).length
             //修正光标位置
             this.selectionEnd = this.selectionStart = caret + curSp - sp
         }
     })
 })
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