Validate html text input as it's typed

Question

What\'s the best way of validating an HTML text input as it\'s typed? All ways I know of doing it has some drawbacks:

1. Using $.keypress you only

Answer 1

Here is the code

function validatephone(xxxxx) 
{
  var maintainplus = '';
  var numval = xxxxx.value
  if ( numval.charAt(0)=='+' )
  {
    var maintainplus = '';
  }
  curphonevar = numval.replace(/[\\A-Za-z!"£$%^&\-\)\(*+_={};:'@#~,.Š\/<>\" "\?|`¬\]\[]/g,'');
  xxxxx.value = maintainplus + curphonevar;
  var maintainplus = '';
  xxxxx.focus;
}