Hi Umbracians

I need help with the output of the checkbox list values.What I want is to add the values as classes for my list... As shown below:

<li><xsl:attribute name="class"><xsl:value-of select="filter"/></xsl:attribute>
      <a href="{umbraco.library:NiceUrl(@id)}">
          <xsl:value-of select="@nodeName"/></h4>
      </a>
  </li>

But the output I get is:

class="class1,class2,class3" 

I need it to be separated with a space instead of a comma, like this:

class="class1 class2 class3"

Can anyone tell me how to do that?