select case when trim(TRANSLATE(col1, '0123456789-,.', ' ')) is null
            then round(to_number(col1)) || 'x' -- numeric
            else col1                          -- alpha
       end
from tab1;

WITH
    TESTDATA
    AS
        (SELECT '1.7313883' AS VALS FROM DUAL
         UNION ALL
         SELECT '1.7313883' FROM DUAL
         UNION ALL
         SELECT '1.63721' FROM DUAL
         UNION ALL
         SELECT 'nm' FROM DUAL
         UNION ALL
         SELECT 'nm' FROM DUAL
         UNION ALL
         SELECT 'nm' FROM DUAL)
SELECT CASE WHEN 
            REGEXP_LIKE(VALS, '([:digit:]|\.)') THEN 
                TO_CHAR(
                    ROUND(
                        TO_NUMBER(
                            REGEXP_REPLACE(VALS, '\.', ',')  -- replacement takes place here
                        ), 2
                    )
                ) || 'x'
            ELSE VALS
       END
           DAT
  FROM TESTDATA;

select case when regexp_like(col, '\d+(\.\d+)?') 
          then to_char(round(to_number(col), 2)) || 'x'
          else col
       end
from  table;