SQL Server 2019

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CREATE TABLE dbo.[Table] (ID int IDENTITY, --As I assume you have some kind of ID column
                         [Column] varchar(100));
INSERT INTO dbo.[Table] ([Column])
VALUES ('test\'),
       ('sample\\'),
       ('example\\\'),
       ('Another\example');
4 rows affected

SELECT [Column]
FROM dbo.[Table];


Column


test\

sample\

example\\

Another\example


UPDATE dbo.[Table]
SET [Column] = REPLACE(REPLACE(REPLACE([Column], '\\', '|'),'\','\\'),'|','\\');
4 rows affected

SELECT [Column]
FROM dbo.[Table];


Column


test\

sample\

example\\

Another\example


TRUNCATE TABLE dbo.[Table];
INSERT INTO dbo.[Table] ([Column])
VALUES ('test\'),
       ('sample\\'),
       ('example\\\'),
       ('Another\example'),
       ('A\harder\\example\'),
       ('A\far\\more\complex\\example\\\than\is\\needed\');
6 rows affected

SELECT *
FROM dbo.[Table];


ID
Column


1
test\

2
sample\

3
example\\

4
Another\example

5
A\harder\example\

6
A\far\more\complex\example\\than\is\needed\


WITH rCTE AS(
    SELECT T.ID,
           T.[Column],
           PI.I,
           STUFF(T.[Column],PI.I+1,0,'\') AS NewColumn,
           1 AS Iteration
    FROM dbo.[Table] T
         CROSS APPLY (VALUES(PATINDEX('%[^\]\[^\]%',[Column])))PI(I)
    UNION ALL
    SELECT r.ID,
           r.[Column],
           PI.I,
           LEFT(r.NewColumn,r.I+1) + STUFF(S.StuffColumn,PI.I+1,0,'\') AS NewColumn,
           r.Iteration+1 AS Iteration
    FROM rCTE r
         CROSS APPLY (VALUES(STUFF(r.NewColumn,1,r.I+1,'')))S(StuffColumn)
         CROSS APPLY (VALUES(PATINDEX('%[^\]\[^\]%',StuffColumn)))PI(I)
    WHERE PI.I > 0)
UPDATE T
SET [Column] = CASE WHEN r.NewColumn LIKE '%[^\]\' THEN r.NewColumn + '\' ELSE r.NewColumn END
FROM dbo.[table] T
     CROSS APPLY (SELECT TOP (1)
                         CASE WHEN r.NewColumn LIKE '%[^\]\' THEN r.NewColumn + '\' ELSE r.NewColumn END AS NewColumn
                  FROM rCTE r
                  WHERE r.ID = T.ID
                  ORDER BY r.Iteration DESC) r;
6 rows affected

SELECT *
FROM dbo.[Table];


ID
Column


1
\test\

2
\sample\

3
\example\\

4
Another\example

5
A\harder\example\

6
A\far\more\complex\example\\than\is\needed\


DROP TABLE dbo.[Table];

ID	Column
1	test\
2	sample\
3	example\\
4	Another\example
5	A\harder\example\
6	A\far\more\complex\example\\than\is\needed\

ID	Column
1	\test\
2	\sample\
3	\example\\
4	Another\example
5	A\harder\example\
6	A\far\more\complex\example\\than\is\needed\