.
with test as(
select '5102-23122910-9-5-6bin.CSV' [a]
union all
select '5102-23122910-9-5-26bin.CSV' [a]
)
select reverse(substring(REVERSE(a),CHARINDEX('.nib',REVERSE(a))+4,CHARINDEX('-',REVERSE(a))-CHARINDEX('.nib',REVERSE(a))-4) )from test