Oracle 11g Release 2

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CREATE TABLE t
    ("CLASS" varchar2(1), "CLASSNUMBER" varchar2(4))
;

INSERT ALL 
    INTO t ("CLASS", "CLASSNUMBER")
         VALUES ('S', 'S3')
    INTO t ("CLASS", "CLASSNUMBER")
         VALUES ('T', 'T37')
    INTO t ("CLASS", "CLASSNUMBER")
         VALUES ('T', 'T50')
    INTO t ("CLASS", "CLASSNUMBER")
         VALUES ('S', 'SS07')
    INTO t ("CLASS", "CLASSNUMBER")
         VALUES ('S', 'S4')
    INTO t ("CLASS", "CLASSNUMBER")
         VALUES ('S', 'SG27')
SELECT * FROM dual
;
6 rows affected

SELECT CLASSNUMBER,REGEXP_REPLACE(ClassNumber, '^'||Class, '') as id FROM t


CLASSNUMBER
ID


S3
3

T37
37

T50
50

SS07
S07

S4
4

SG27
G27