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I believe column E is under-constrained; a solution with column E = "YYYY " or "OOOO " passes the tests, but is clearly not what's intended.

Never mind, this was an error in the https://regexcrossword.com/playerpuzzles/595e5542d2433 version, which has a (.) where it should have a \1 in row 2 (thanks to @angry_albatross).

In the expression in the second row, the 5th character must match the 12th character, which must be R.