Had to find some strings (varchar2) in a table that were just blanks with optional end-of-line characters thrown in.
Luckily, regular expressions make that an easy task. Here is the query:
SELECT * FROM myTable x WHERE REGEXP_LIKE( x.myColumn, '(^[[:space:]]*$)' );
A short explanation:
^...$says that pattern applies to the string from start to finish, i.e., it’s not just a sub-string pattern,
[...]*says that pattern occurs 0 or more times (could also have been […]+ in this case),
[:space:]defines a pattern of all white-space characters, including blank, \t, \r and \n.
Sometimes, regular expression just make tasks like these very easy.