SQL & PL/SQL

Hi,

select regexp_substr('What a good day for (YOU [ME] and HIM','[:ALNUM:]*[^\(]*') from dual;
result is:
SQL> What a good day for

but why this pattern does not work for ']' when I replace '(' with ']'?
select regexp_substr('What a good day for (YOU [ME] and HIM','[:ALNUM:]*[^\]]*') from dual;
result is:
SQL> W

I need to get a sub-string before either '(', '[', '{' , ')', ']', or '}' in a give string, this pattern works for '(', ')', '{', '}' but not '[' or ']'
what should the pattern be? If '[:ALNUM:]*[^\(]*' is not correct?

Can anyone help?

thanks
Fiona