SQL Server 2022

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CREATE TABLE #t (Code VARCHAR(MAX),S VARCHAR(MAX));
INSERT INTO #t (Code ,S )
VALUES ('1','\String\ String'),
        ('2','Laser \Laser\'),
        ('3', 'La\ser\s and \S\trings'),
        ('4', 'Las\ and String'),
        ('5', 'Las\ and String\'),
        ('6', 'Las and String\'),
        ('7', '\Las and String'),
        ('8', '\Las and String\'),
        ('9', '\L\as and String\')
  ;
SELECT *
FROM   #t;


Code
S


1
\String\ String

2
Laser \Laser\

3
La\ser\s and \S\trings

4
Las\ and String

5
Las\ and String\

6
Las and String\

7
\Las and String

8
\Las and String\

9
\L\as and String\


with r as (
  select 0 lvl, code,s
    ,case when CHARINDEX('\', s)>0 and  CHARINDEX('\', s, CHARINDEX('\', s) + 1)>0 then
        substring(s,1,CHARINDEX('\', s)-1)
       +substring(s,CHARINDEX('\', s, CHARINDEX('\', s) + 1)+1,len(s)) 
     else s
     end rest
  from #t
  union all
  select r.lvl+1,code,s
    ,case when CHARINDEX('\', rest)>0 
            and  CHARINDEX('\',rest, CHARINDEX('\',rest) + 1)>0 then
        substring(rest,1,CHARINDEX('\', rest)-1)
       +substring(rest,CHARINDEX('\', rest, CHARINDEX('\', rest) + 1)+1,len(s)) 
     else rest
     end rest
  from r where CHARINDEX('\', rest)>0 and CHARINDEX('\',rest, CHARINDEX('\',rest) + 1)>0
)
update #t
  set s=rest
from #t t
left join(
select * ,max(lvl)over(partition by code) rn
from r)src on src.code= t.code and src.rn=src.lvl
9 rows affected

select * from #t;


Code
S


1
 String

2
Laser

3
Las and trings

4
Las\ and String

5
Las

6
Las and String\

7
\Las and String

8


9
as and String\

Code	S
1	\String\ String
2	Laser \Laser\
3	La\ser\s and \S\trings
4	Las\ and String
5	Las\ and String\
6	Las and String\
7	\Las and String
8	\Las and String\
9	\L\as and String\

Code	S
1	String
2	Laser
3	Las and trings
4	Las\ and String
5	Las
6	Las and String\
7	\Las and String
8
9	as and String\