Find an unmatched curly brackets in a string in SQL - sql

Question

How can I find an unmatched curly brackets in a string in SQL?
DECLARE #iVariable varchar(100)
SET iVariable = '{Day}{Month}{Year}'
If any unmatched left bracket found ({Day}{Month{Year}) then should return 'Unmatched {'
else any unmatched right bracket found ({Day}{Month}Year}) then should return 'Unmatched }'
If there is no unmatched brackets it shoud return the value in comma seperated format eg ('Day,Month,Year')
Is there any logic to do this?

Answer 1

What I would do is that I would validate the length of the string after replacing '{' with '' (empty string).
DECLARE #iVariable varchar(100) = '{Day}{Month}{Year}'
select case
when len(#iVariable) - len(replace(#iVariable, '{', '')) < len(#iVariable) - len(replace(#iVariable, '}', ''))
then 'Unmatched }'
when len(#iVariable) - len(replace(#iVariable, '{', '')) > len(#iVariable) - len(replace(#iVariable, '}', ''))
then 'Unmatched {'
else right(replace(replace(#iVariable, '{', ','), '}', ''), len(replace(replace(#iVariable, '{', ','), '}', '')) - 1)
end
What happens here is that I check if there are more '}' than '{', it returns unmatched '}'.
Likewise for '}'.
If the number matches, it returns the original string, with '{' and '}' replaced out, and commas inserted instead.
Edit: As Gordon stated in the comments, this does not work for for example '{}}{'.
Instead, You could use a user defined function. For instance:
create function SomeFunc(#iVariable varchar(2000))
returns varchar(3000)
as
begin
if len(replace(replace(#iVariable, '}', ''), '{', '')) = 0
return 'No data present'
else
begin
-- Declare stuff to be used
declare #result varchar(3000) = ''
declare #AMT_Left int = len(#iVariable) - len(replace(#iVariable, '{', ''))
declare #AMT_Right int = len(#iVariable) - len(replace(#iVariable, '}', ''))
-- First test if no. of brackets match:
if #AMT_Left > #AMT_Right
set #result = 'Unmatched }'
else if #AMT_Left < #AMT_Right
set #result = 'Unmatched {'
else if #AMT_Left = #AMT_Right
begin
-- If matched, define result, and use while loop for error handling
set #result = right(replace(replace(#iVariable, '{', ','), '}', ''), len(replace(replace(#iVariable, '{', ','), '}', '')) - 1)
DECLARE #intFlag INT
SET #intFlag = 1
-- Loop through each set and check if '{' occurs before '}':
WHILE (#intFlag <= #AMT_Left and #result != 'Non matching pair')
BEGIN
if charindex('{', #iVariable) > charindex('}', #iVariable)
set #result = 'Non matching pair'
set #iVariable = right(#iVariable, len(#iVariable) - charindex('}', #iVariable))
SET #intFlag = #intFlag + 1
end
end
end
return #result
end;
go
Testing with these input values:
select dbo.SomeFunc('{Day}{Month}{Year}')
select dbo.SomeFunc('{Day}{Month{Year}')
select dbo.SomeFunc('{Day}{Month}Year}')
select dbo.SomeFunc('{}{}')
select dbo.SomeFunc('{}}{')
select dbo.SomeFunc('{Day}}Month{')
result:
Day,Month,Year
Unmatched }
Unmatched {
No data present
No data present
Non matching pair

Answer 2

There may be a more elegant way to do this, but the following should capture all the cases:
with v as (
select '{Day}{Month}{Year}' as var union all
select '{Day}{Month}{Year}}{' union all
select '{Day}{Month}{Year}}}'
),
cte as (
select left(var, 1) as c, 1 as num, var
from v
union all
select substring(var, num+1, 1), num + 1, var
from cte
where num <= len(var)
)
select var,
(case when min(balance) < 0 then 'Unbalanced }'
when sum(case when c = '{' then 1
when c = '}' then -1
else 0
end) > 0
then 'Unbalanced {'
else 'Balanced'
end)
from (select cte.*,
(select sum(case when c = '{' then 1
when c = '}' then -1
else 0
end)
from cte cte2
where cte2.var = cte.var and cte2.num <= cte.num
) as balance
from cte
) t
group by var;
This explodes the values character by character and then checks for the balance.