Different class for each extra field

[susawx +]

Hi! How can I add a class to each extra field row? I've added the title as a class in viewlisting.tpl like this:
{* Extra Fields *}
{foreach from=$extrafields item=extras}
<tr class="{$extras.title}">

but my titles has �, � and many blank spaces to use it as a class name. How can I add code to generate a different class for each one? Like class="extra1", class="extra2", etc?
Thanks!!