Error "ORA-12726:unmatched bracket in regular expression" while trying to separate a part of a string from other part I have a string like this :

v_sgtring:'sum:[sum(x)&gt;1 and sum(y)&gt;2 and sum(z)&gt;3],where:[n=1 and m=2]'


and I need to separate sum:[] from where:[] and assign them to different variables like this:

v_sum_string:'sum(x)&gt;1 and sum(y)&gt;2 and sum(z)&gt;3';
v_where_string:'n=1 and m=2';


What I've written so far is:

select regexp_substr('sum:[sum(x)&gt;1 and sum(y)&gt;2 and sum(z)&gt;3],where:[n=1 and m=2]',
                 'sum:[]+,',
                 1,
                 1)
from dual;


but it gives me error ORA-12726:unmatched bracket in regular expression and I can not understand where my problem is .
I was wondering if you could help me with this and if you have better opinion about the original string (for example it would  be easier to separate if it was parentheses) please say 

thanks very much in advance
The error is because [ and ] are part of regex syntax, which makes handling them in a regex tricky as you have to escape them, e.g. 'sum:\[\]+,'
I found it easier to replace them on the fly with { and } before applying regexp_substr to the result:
with demo (str) as
      ( select 'sum:{sum(x)&gt;1 and sum(y)&gt;2 and sum(z)&gt;3},where:{n=1 and m=2}'
        from dual )
select regexp_substr(translate(str,'[]','{}'),
         '(sum:{)([^{}]+)', 1, 1, 'i', 2) as sum_expr
     , regexp_substr(translate(str,'[]','{}'),
         '(where:{)([^{}]+)', 1, 1, '', 2) as where_expr
from   demo;

SUM_EXPR                            WHERE_EXPR
----------------------------------- -----------
sum(x)&gt;1 and sum(y)&gt;2 and sum(z)&gt;3  n=1 and m=2