How do I prevent user from entering specific characters in a textbox using jQuery?

Try using the fromCharCode method:

$(document).ready(function () {
  $('#tb1').keydown(function (e) {

    var k = String.fromCharCode(e.which);

    if (k.match(/[^a-zA-Z0-9]/g))
      e.preventDefault();
  });
});

You use keypress rather than keydown and prevent the default action.

For example, this prevents typing a w into the text input:

$("#target").keypress(function(e) {
  if (e.which === 119) { // 'w'
    e.preventDefault();
  }
});

Live Copy | Source

Update: If it's applying the regex that's giving you trouble:

$("#target").keypress(function(e) {
  if (String.fromCharCode(e.which).match(/[^A-Za-z0-9 ]/)) {
    e.preventDefault();
  }
});

Live Copy | Source