Assuming 0 indexing: row 0 column 4. Constrained by rows 0, 2, 3 where rows 0 and 2 have [^XZVCHFJLQM] and [^\sPQFB] and row 3 has [OYSRU]. All of: O, Y, S, R and U match. Am I missing something?
In row 2, column 4 the (.) is reused via \1 in column 11 where it has to be R.
You did the same thing I did I imagine. The regexcrossword.com link has a small mistake: