If we're being picky (and I do love to be picky), you should add the sticky, setuid, and setgid flags too.
Resulting in:
^[-r][-w][-xXs][-r][-w][-xXs][-r][-w][-xXt]$
(The capital letter X is where...
First, your input (obviously a filemode mask) has more structure: the "r" can only appear in the 1nd, 4th or 7th place, the "w" can only appear in the 2rd, 5th or 8th place, etc.