I read your edit ... it seems like you are searching for any uppercase letter anywhere in the string, but ignoring lowercase letters. If that's the case, this would be the key condition:
where compress(x, , 'kU') > ' ';
Within the COMPRESS function, the third parameter is specifying k=KEEP rather than remove and U=Uppercase letters.
If you want to locate any letter (whether uppercase or lowercase) it would be:
where compress(x, , 'ka') > ' ';
Of course, the SQL interpreter will need to understand how to use COMPRESS.
And if you want a subset of the letters, you would still have to spell them out:
where compress(x, 'ABCDEFGHIJ', 'k') > ' ';
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