I think the majority of the body of literature written by Egyptians is
written in formal Arabic. I simply come to this conclusion because as an
avid reader I must have come across only one or two literary pieces written
in Egyptian Arabic as 'pioneering experimental' works (as one author called
his stuff).
For the sake of correctness, I should state that my field of reading
includes history, science, political analysis and generally non-fiction
topics, I have just been reminded that some poets that use Egyptian as their
writing language (it is a sub-genre called A'mmy), however they didnt have a
standard agreed-upon way of writing it too.
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Best Regards,
Muhammad Alsebaey