Ajax form validation is one of those features typically tasked to a Drupal Developer. Here is how it is accomplished in Drupal 6.

Let's say we were adding Ajax form validation to the user registration page. You need to create a jQuery event listener in a javascript file.

$(document).ready(function(){    
  $("#edit-name").blur(function () {
    $.post('modulename/validate', { name: this.value }, function(data) {
      $('#edit-name-wrapper .description').append(data);
    });
  });
 });

What this does is when the user clicks away from the username field it takes the value of that field and does an HTTP POST to modulename/validator (we will actually set up this url in a minute). If the name isn't valid, or we need to display some sort of error message, we append this to the description, or you could just define a new div to place stuff into.

Now, let's set up our custom module. You need to create a modulename directory in sites/all/modules then inside that directory create 2 files modulename.module and modulename.info with a little bit of information about your module. See http://drupal.org/node/206756 for reference. Now open up your .module file and let's create the url.

/**
* Implementation of hook_menu().
*/
function modulename_menu () {
  $items['modulename/validate'] = array(
    'page callback' => "modulename_ajax_validate",
    'page arguments' => array(2),
    'type' => MENU_CALLBACK,
  );
  
  return $items;
}

function modulename_ajax_validate ($string = '') {
  if ($string) {
    //do your validation here...say we didn't want to allow underscores in the username
    if(strstr($string,"_")) {
      $errors[] = t("Underscores in your username aren't allowed.");
    }
    foreach ($errors as $error) {
      print "$error <br />";
    }
  }
}

What this does is creates a path for the validation (modulename/validate) and then calls a function (there's nothing special about this function name, you could have called it foo_validateme) passing it the string parameter parameter. Then if the validation fails, it adds a string to the errors array. If there are errors, loop through the errors and print them out. This string gets passed back to our jQuery handler we wrote above as the variable name: data and gets printed out and appended to the description of the username field.

You could try this example, but it also might be prudent to have a look at the Ajax Project on Drupal.org.

Dear fellow Drupal Developer, you are now armed with the awesomeness of Ajax. Happy validating!

UPDATE: Several readers have recommended using the jQuery Validation plugin. This is a great plugin and have used it many times. I suggest you check it out. Also, there is a good example of doing AHAH in the examples module on drupal.org. Another reader commented that it would be a great idea to use the same validation function name so that the form would be validated by the same code using Ajax or just normal form submission. For this to work, you would need to get the name right using the Drupal Form API. Here is an example of how to do that.