Convert varchar DD-MM-YYYY or DD/MM/YYYY to YYYY-MM-DD in SQL Query

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declare @DateStr varchar(100)=

'13-09-2019' --or '13/09/2019'

or '2019-09-13'

or 

SELECT case when len(isnull(@DateStr ,''))>0 then CONVERT(VARCHAR(10), case when substring(@DateStr ,5,1)='-' or substring(@DateStr ,5,1)='/'then @DateStr else CONVERT(date, @DateStr , 105) end, 23) else null end

//////////////////// Result

2019-09-13 or 

2019-09-13 or  

2019-09-13 or 

NULL