Zend_Filter provides a library of static methods for filtering data. For input filtering, you should use Chapter 7, Zend_InputFilter instead, because it provides a framework for filtering input using the methods provided by this class. However, because Zend_InputFilter is designed primarily for arrays, Zend_Filter can be useful for filtering scalars, because it behaves like PHP's string functions:

<?php

$alphaUsername = Zend_Filter::getAlpha('John123Doe');

/* $alphaUsername = 'JohnDoe'; */

?>