select nodes with xpath, having by specific attribute named 'class' the 'subvalue part' set

<style>
   .marek{}
</style>
etc..
<root>
<element class="marek mand"/>
<element class="mand marek"/>
<element class="mand marekmänd"/>
<element class="mand marekk"/>
</root>

I would like to know what is the XPath expression for Mozillas 
JavaScript document.evaluate method first argument, if i want to get all 
the nodes, that have the 'css'-class named "marek" set.

By above data only 2 elements should be returned, because they and they 
only have the 'css' class "marek" set. The XPath expression should not 
match 'marekk' nor 'marekmänd'.


I read few articles about xpath and i didnt saw any regular expressions 
to be used there, but what i saw was that the xpath string functions are 
definately too weak and clumsy to achieve my goal. I am new to XPath, so 
i thought some master regular could teach me a bit.

TIA