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The regular expression contains an unspecified Perl extension error. Any idea?
(?i)(?u)[\x{0419}\x{0041}]


and incorrect pattern
[\x{0419}\x{0041}]\u
[\x{0419}\x{0041}]\l

Hi.. Unfortunately (?u) is not a valid Perl extension for either PCRE or PCRE2.
But the format: \x{###} suffices for matching unicodes (leading 0's can be omitted).

Same thing with \u for uppers-only and \l for lowers-only (neither PCRE/PCRE2).
But you can always use (?-i) to terminate (?i)'s case-insensitivity.
Cheers!