Is there an inbuilt DB2 function or any query to check if the character i have is a number? (I cannot use user defined functions)
There are many approaches. Take a look at that solution using only two functions:
CASE WHEN REPLACE(TRANSLATE(test_str, '0','123456789','0'),'0','') = '' THEN 'All digits' ELSE 'Not all digits' END
In general - less functions - better performance :)