Is it possible to find the first occurrence of a string that's NOT within a set of delimiters in SQL Server 2016+?

I have a column in a SQL Server table that has strings of varying lengths. I need to find the position of the first occurrence of the string , -- that's not enclosed in single quotes or square brackets.

For example, in the following two strings, I've bolded the portion I would like to get the position of. Notice in the first string, the first time , -- appears on its own (without being between single quote or square bracket delimiters) is at position 13 and in the second string, it's at position 16.

'a, --'[, --]**, --**[, --]

[a, --b]aaaaaaa_ **, --**', --'

Also I should mention that , -- itself could appear multiple times in the string.

Here's a simple query that shows the strings and my desired output.

SELECT 
    t.string, t.desired_pos
FROM
    (VALUES (N'''a, --''[, --], --[, --]', 14),
            (N'[a, —-b]aaaaaaa_ , --'', --''', 18)) t(string, desired_pos)

Is there any way to accomplish this using a SELECT query (or multiple) without using a function?

Thank you in advance!

I've tried variations of SUBSTRING, CHARINDEX, and even some CROSS APPLYs but I can't seem to get the result I'm looking for.

Try this approach. I'm replacing the strings you don't need for another string of the same length. Then look for the position of the interested string.

  SELECT string, desired_pos, 
         CHARINDEX(', --', REPLACE(REPLACE(string, ''', --''', '******'), '[, --]', '******') 
         ) start_index
  FROM (VALUES (N''', --''[, --], --[, --]', 13),
               (N'[, --]aaaaaaa_ , --'', --''', 16)) t(string, desired_pos)

I don't know if it makes sense with a C# solution, but this class for CVS is a nice little parcer: TextFieldParser

Then you just define Delimeters etc. and assuming the input is escaped consistently then all is good.

Before i write down my solution, i must warn you: DON'T USE IT. Use a function, or do this in some other language. This code is probably buggy.
It doesn't handle stuff like escaped quotes etcetc.

The idea is to first remove the stuff inside brackets [] and quotes '' and then just do a "simple" charindex.
To remove the brackets, i'm using a recursive CTE that loops ever part of matching quotes and replaces their content with placeholder strings.

One important point is that quotes might be embedded in each other, so you have to try both variants and chose the one that is earliest.

WITH CTE AS (
	SELECT	*
	FROM
	(VALUES (N'''a, --''[, --], --[, --]', 14),
            (N'[a, —-b]aaaaaaa_ , --'', --''', 18)) t(string, desired_pos)
           )
, cte2 AS (
	select	x.start
	,	x.finish
	,	case when x.start > 0 THEN STUFF(string, x.start, x.finish - x.start + 1, REPLICATE('a', x.finish - x.start + 1)) ELSE string END AS newString
	,	1 as level
	,	string as orig
	,	desired_pos
	from	cte
	CROSS APPLY (
		SELECT	*
		,	ROW_NUMBER() OVER(ORDER BY case when start > 0 THEN 0 ELSE 1 END, start) AS sortorder
		FROM	(
			SELECT	charindex('[', string) AS start
			,	charindex(']', string) AS finish
			UNION ALL
			SELECT	charindex('''', string) AS startQ
			,	charindex('''', string, charindex('''', string) + 1) AS finishQ
		) x
	) x
	WHERE	x.sortorder = 1
	
	UNION ALL
	select	x.start
	,	x.finish
	,	STUFF(newString, x.start, x.finish - x.start + 1, REPLICATE('a', x.finish - x.start + 1))
	,	1 as level
	,	orig
	,	desired_pos
	from	cte2
	CROSS APPLY (
		SELECT	*
		,	ROW_NUMBER() OVER(ORDER BY case when start > 0 THEN 0 ELSE 1 END, start) AS sortorder
		FROM	(
			SELECT	charindex('[', newString) AS start
			,	charindex(']', newString) AS finish
			UNION ALL
			SELECT	charindex('''', newString) AS startQ
			,	charindex('''', newString, charindex('''', newString) + 1) AS finishQ
		) x
	) x
	WHERE	x.sortorder = 1
	AND	x.start > 0
	AND	cte2.start > 0 -- Must have been a match
)

SELECT	PATINDEX('%, --%', newString), *
from (
	select *, row_number() over(partition by orig order by level desc) AS sort
	from cte2
	) x
where x.sort = 1

Im late the game here but This kind of thing is simple in SQL Server when leveraging NGrams8k. Not only do you not need REGEX, a CLR, C# required. Furthermore, NGrams8k will be the fastest by far. In 8 years nobody has produced anything remotely as fast. Furthermore, this code will be faster and far less complex than a recursive CTE solution (which are almost always slow in SQL Server)

;--==== Sample Data
DECLARE @T Table (String VARCHAR(100))
INSERT @T
VALUES (N'''a, --''[, --], --[, --]'),
(N'[a, —-b]aaaaaaa_ , --'', --''');
;--==== Solution
SELECT
t.String, ng.Position
FROM        @t                                       AS t
CROSS APPLY (VALUES(REPLACE(t.String,'[',CHAR(1))))  AS f(S)
CROSS APPLY samd.NGrams8k(f.S,4)                     AS ng
CROSS APPLY (VALUES(SUBSTRING(f.S,ng.Position-2,7))) AS g(String)
WHERE ng.Token = ', --'
AND  g.String NOT LIKE '%''%''%'
AND  g.String NOT LIKE '%'+CHAR(1)+'%]%';

Results:

String                        Position
----------------------------- --------------------
'a, --'[, --], --[, --]       14
[a, —-b]aaaaaaa_ , --', --'   18