WITH t AS
(
SELECT '6"' size_tx FROM dual
UNION ALL SELECT '50A'    FROM DUAL
UNION ALL SELECT '3/4"'   FROM DUAL
UNION ALL SELECT '1.1/2"' FROM DUAL
)
SELECT size_tx
     , REGEXP_REPLACE(size_tx, '[0-9/.]') regexp_1
     , REGEXP_REPLACE(size_tx, '[^0-9/.]') regexp_2
     , TRANSLATE(size_tx, 'a0123456789/.', 'a') trans_1
     , TRANSLATE(size_tx, '0123456789/.' || size_tx, '0123456789/.') trans_2
  FROM t
;