$("#myinput").keyup(function(){

  // prevent every character except numbers

  if(!this.value.slice(-1).match(/^[0-9]+\.?[0-9]*$/) ){

    this.value = this.value.slice(0, -1);

    return;

  }

  // remove the dots, split the string and reverse it

  var a = this.value.replace(/\./g, '').split('').reverse();

​

  // start from 3 and as long as there's a number 

  // add a dot every three digits.

  var pos = 3;

  while(a[pos] !== undefined){

    a.splice(pos,0,'.');

    pos = pos + 4;

  }  

  // reverse, join and reassign the value

  this.value = a.reverse().join('');

});

​