Strong Password Validation. This was used by a gov't site that needed to use these rules.

gistfile1.php

<?php
  /**
   * Rule: strongPassword. Client requirements for a strong password
   * 
   * @param  string  $field password
   * @return boolean
   */
  public function strongPassword($field) {
    // Set RegEx values
    $rule_uppercase = '/[A-Z]/';
    $rule_lowercase = '/[a-z]/';
    $rule_nonalpha  = '/[(!@#$%^&*_+=?~`;:,<>|)0123456789]/';

    // No non-alpha characters in first or last position
    if (strpos($rule_nonalpha, substr($field, 0, 1)) !== FALSE or strpos($rule_nonalpha, substr($field, -1)) !== FALSE) {
      return FALSE;
    }
    
    // At least one uppercase letter
    if (preg_match_all($rule_uppercase, $field, $match) < 1) {
      return FALSE;
    }

    // At least one lowercase letter
    if (preg_match_all($rule_lowercase, $field, $match) < 1) {
      return FALSE;
    }

    // At least 2 non-alpha characters
    if (preg_match_all($rule_nonalpha, $field, $match) < 2) {
      return FALSE;
    }
    return TRUE;
  }