CREATE OR REPLACE FUNCTION find_a_numbers(string_ IN VARCHAR2) RETURN NUMBER
IS
return_ VARCHAR2(4000);
validchars_ VARCHAR2(10):= '0123456789';
found_ BOOLEAN;
len_ NUMBER:=length(string_);
i_   NUMBER;
v    NUMBER;
BEGIN
IF nvl(len_,0)>0 THEN
FOR i_ IN 1..len_ LOOP
found_ := FALSE;
FOR v IN 1..length(validchars_) LOOP
IF substr(string_,i_,1)=substr(validchars_,v,1) THEN
found_ := TRUE;
EXIT;
END IF;
END LOOP;
IF found_ THEN
return_ := return_ ||substr(string_,i_,1);
END IF;
END LOOP;
END IF;
RETURN to_number(nvl(return_,'0'));
END find_a_numbers;

SELECT find_a_numbers('9876OMER123232465') FROM dual;
9876123232465
 
SELECT find_a_numbers('ORAcle12345+67') FROM dual;
1234567
 
SELECT find_a_numbers('34464565') FROM dual;
34464565
 
SELECT find_a_numbers('xyz+dfgfd') FROM dual;
0