I finally found the following:
REPLACE ALL OCCURRENCES OF REGEX '^\"|\"$' IN TABLE it_file_header WITH ''
But I have another serious problem. 4 out of 5 ITABs have amount fields and I am getting the following error:
The table "A" expects a character-like field (in CHAR MODE) or a byte-like field (in BYTE MODE) as its row type.
And I want to avoid LOOPs
Matthew, Sandra, you misunderstood me. I didn't say to not use LOOPs but to minimize the use especially if ITABs have a lot of records, like mine. I have 5 ITABs with this problem in all character fields and four of them have amount fields and these 4 have more than 100000 records. That's why I wanted to avoid LOOPs.