$(function() {

  // Proof of concept
  var handleCurrentInput = function (e) {

    var $this = $(this),
        valStr = $this.val(),
        valArr, lastIndexAt, lastItem;

    if ( e.shiftKey && e.which === 50 ) {

      lastIndexAt = valStr.lastIndexOf('@');
      valArr  = valStr.split('@');
      lastItem  = valArr.pop();

      // fake logic that pretends to be twitter user autocomplete
      valArr.push('@user ')
      valArr.push(lastItem);

      $this.val(valArr.map(function(str) {
        return $.trim(str);
      }).join(' '));
    }

  };


  $('textarea')
    .bind('keyup', handleCurrentInput)
    .trigger('focus');


});