For illustration purposes, a few test cases follow…

-- TRUE
EXEC IF is_numeric('0123') THEN DBMS_OUTPUT.PUT_LINE('TRUE'); END IF;
-- TRUE
EXEC IF is_numeric(0.123) THEN DBMS_OUTPUT.PUT_LINE('TRUE'); END IF;
-- TRUE
EXEC IF is_numeric('0.123') THEN DBMS_OUTPUT.PUT_LINE('TRUE'); END IF;
-- TRUE
EXEC IF is_numeric(123) THEN DBMS_OUTPUT.PUT_LINE('TRUE'); END IF;
-- TRUE
EXEC IF is_numeric('123') THEN DBMS_OUTPUT.PUT_LINE('TRUE'); END IF;
-- FALSE
EXEC IF NOT is_numeric('foo') THEN DBMS_OUTPUT.PUT_LINE('FALSE'); END IF;

FUNCTION is_numeric(number_str VARCHAR2) RETURN BOOLEAN IS
x_is_numeric PLS_INTEGER := 0;
BEGIN
SELECT XMLELEMENT (”ROWSET”,
XMLAGG (
XMLELEMENT(”ROW”, xmlattributes(ROWNUM AS “num”),
XMLELEMENT (”STRING_VAL”, REPLACE(number_str, ‘,’, ‘.’))
))
).transform(XMLTYPE.createXML(
‘’ ||
‘’ ||
‘’ ||
‘’ ||
‘’ ||
‘ ‘ ||
‘ ‘ ||
‘ ‘ ||
‘’ ||
‘’ ||
‘’ ||
‘’)).getStringVal() AS RES
INTO x_is_numeric
FROM dual;
RETURN x_is_numeric > 0;
END is_numeric;

POSSIBLE_NUM N
——————– —
+123.123E+2E-3 NO –> NOT a Valid Number ???
-abcde NO
123 YES
-1.2e-3 YES
abc NO
-123 YES
+123 YES
-123.123 YES
+123.123 YES
-123.123E-23 YES
+123.123E+2 YES

11 rows selected.

10:29:35 server-user@db> select possible_num,
10:31:58 2 case when regexp_like(possible_num,’^[+-]?(\d+\.?|\d*\.\d+)([eE][+-]\d+)?$’)
10:31:58 3 then to_number(possible_num)
10:31:58 4 end n
10:31:58 5 from t;

POSSIBLE_NUM N
——————– ———-
+123.123E+2E-3 –> NOT a Valid Number ???
-abcde
123 123
-1.2e-3 -.0012
abc
-123 -123
+123 123
-123.123 -123.123
+123.123 123.123
-123.123E-23 -1.231E-21
+123.123E+2 12312.3

11 rows selected.

I also forget that 1e10 is valid too, so the expression will be

regexp_like(:x, ‘^[+-]?(\d+\.?|\d*\.\d+)([eE][+-]?\d+)?$’)

SQL> select col2
2 ,decode (length (translate (trim (col2), ‘ +-.0123456789eE’, ‘ ‘) )
3 ,null, ‘Yes’
4 ,’No’) is_number
5 from t;

COL2 IS_
—————————— —
+123.123E+2E-3 Yes
-abcde No
123 Yes
-1.2e-3 Yes
abc No
-123 Yes
+123 Yes
-123.123 Yes
+123.123 Yes
-123.123E-23 Yes
+123.123E+2 Yes

11 rows selected.

I like your options, I just changed a little bit the regexp solution to include the (+) plus sign “[+-]”

SQL> ed
Wrote file afiedt.buf

1 select col2,
2 case when
3 regexp_like(col2,
4 ‘^[+-]?(\d+\.?|\d*\.\d+)([eE][+-]\d+)?$’)
5 then to_number(col2)
6 end n
7* from t
SQL> /

COL2 N
—————————— ———-
+123.123E+2E-3
-abcde
123 123
-1.2e-3 -.0012
abc
-123 -123
+123 123
-123.123 -123.123
+123.123 123.123
-123.123E-23 -1.231E-21
+123.123E+2 12312.3

11 rows selected.

select x, 
  xmlquery('let $i:=if($X castable as xs:decimal)then "YES" else "NO" return $i' 
    passing x as x 
    returning content) isnumeric 
from t;
X       ISNUMERIC
------- ---------
123     YES
-1.2e-3 YES
abc     NO