function valid_tag_expression(a_tags in varchar2) return number is

t_left_side ut_varchar2_list := ut_varchar2_list('|','&',',');

t_right_side ut_varchar2_list := ut_varchar2_list('!','-');

l_left_side_expression varchar2(100) := '[|&,]';

l_left_side_regex varchar(400) := '([^|&,]*)[|&,](.*)';

l_left_side varchar2(4000);

l_rigth_side_expression varchar2(100) := '[!-]';

l_right_side_regex varchar(400) := '([!-])([^!-].*)';

l_right_side varchar2(4000);

l_tags varchar2(4000) := a_tags;

l_result number :=1;

begin

--Validate that we have closed up all brackets

if regexp_count(l_tags,'\(') <> regexp_count(l_tags,'\)') then

l_result := 0;

end if;

--Remove brackets as we dont evaluate expression only validate.

l_tags := replace(replace(l_tags,'('),')');

--Check if there are any left side operators for first in order from left to right

if regexp_count(l_tags,l_left_side_expression) > 0 then

--Extract left part of operator and remaining of string to right

l_left_side := regexp_replace(l_tags,l_left_side_regex,'\1');

l_right_side := regexp_replace(l_tags,l_left_side_regex,'\2');

--If left side is null that means that we used left side operator without

-- left and right e.g. &test

if l_left_side is null then

l_result := 0;

else

--Extract right side from left side expression if there is any !-

--Remove first negation tag to see if there is double negation

l_left_side := regexp_replace(l_left_side,l_right_side_regex,'\2');

end if;

--check that on right side there is no extra negation

if regexp_count(l_left_side,l_rigth_side_expression) > 0 then

l_result := 0;

end if;

--Now process right side of string

if l_right_side is not null then

l_result := least(l_result,valid_tag_expression(l_right_side));

else

l_result := 0;

end if;

else

--We just process single tag.

l_left_side := l_tags;

l_left_side := regexp_replace(l_left_side,l_right_side_regex,'\2');

if regexp_count(l_left_side,l_rigth_side_expression) > 0 then

l_result := 0;

end if;

end if;

return l_result;

end;