<?php
plugin_add_hook( 'CUSTOM_FIELD_VALIDATE', 'plugin_custom_field_validator');

function plugin_custom_field_validator( $array) 
{
   list( $err, $field_id, $field_name, $field_value) = $array;
   if ( $field_name == 'City')
   {
      $f_test = strtolower( trim( $field_value));
      if ( $f_test == 'city' || $f_test == 'yes')
      {
         return 'ERROR';
      }
   }
   return 0;
}

function custom_field_validator_info() 
{
    return array( 'name' => 'Custom Field Validator',
                  'desc' => 'Validate custom profile fields.',
                  'version' => '1.0');
}
?>