Originally Posted by borked

If the minus sign isn't a metacharacter, then what is it doing in eg A-Z and a-z and 0-9???

Of course it needs escaping

^[A-Za-z0-9\.\-_ ]{3,32}$

or shorter still
^[\w\.\-]{3,32}$
note with this though, \w as it's recognising word characters, will allow tabs and line breaks, not just white space.... if you don't want them, negate them:
^[\w\.\-^\t^\n]{3,32}$