How to find alphanumeric characters (a-z, A-Z, 0-9)

By using '[[:alnum:]]' with regular expressions it is possible to find 

alphanumeric characters.By adding {3} at the end we limit the length of 

matching alphanumeric characters.

Select * from test_table where REGEXP_LIKE(test_column, '[[:alnum:]]');

Select * from test_table where REGEXP_LIKE(test_column, '[[:alnum:]]{3}');