manipulating variables in templates

[SOL00]

hello,

i would like to manipulate variables in a template.
for example:

$thread[preview] gives me a preview of the thread. but i would like to have this preview on 1 line, so without any enters in it.

is there any way to accomplish this? something like:
<expression var="$thread[preview]" action="nl2br" print="true" />
??

I'm you guessing here, but this would be an interesting way to do this.
hope anybody can help me.

Grtz