In Question preview window, the pmatch question does not Fill in correct response(s). It simply fills the answer field with the first Answer, which is obviously a regular expression, not a valid answer.
I suggest looking at the (different) ways that the preg and the regexp question types solve this problem, by providing a "best possible answer" to be used in this case.
Note, this same unavailability of a "best correct answer" means that a "correct answer" in never displayed to the students in Adaptive mode.