[rootlocus]

I'm assuming the solution isn't unique because I found some positions that are under-constrained.

[vhold]

I only found one column to have multiple solutions, and saved it for last, at which point only one option made sense.

[angry_albatross]

Which positions? I didn't find any that were under-constrained.

[rootlocus]

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?

[jgrahamc]

In row 2, column 4 the (.) is reused via \1 in column 11 where it has to be R.

[70jS8h5L]

You did the same thing I did I imagine. The regexcrossword.com link has a small mistake:

[^PZVJG]{4}(.)[EFUG]{6}(.)[^\sPZVJI]{2}

should be

[^PZVJG]{4}(.)[EFUG]{6}\1[^\sPZVJI]{2}

(note the \1 )