PHP regex: what is "class at offset 0"?

I'm trying to strip all punctuation out of a string using a simple regular expression and the php preg_replace function, although I get the following error:

Compilation failed: POSIX named classes are supported only within a class at offset 0

I guess this means I can't use POSIX named classes outside of a class at offset 0. My question is, what does it means when it says "within a class at offset 0 "?

$string = "I like: perl";    

if (eregi('[[:punct:]]', $string))  
    $new = preg_replace('[[:punct:]]', ' ', $string); echo $new;