I used a lengthy RegEx to find all of the permutations of the word CLIMATE in the New York Times Crossword database. It works, but may be too long:
88 results for regular expression ([CLIMATE])(?!\1)([CLIMATE])(?!\1|\2)([CLIMATE])(?!\1|\2|\3)([CLIMATE])(?!\1|\2|\3|\4)([CLIMATE])(?!\1|\2|\3|\4|\5)([CLIMATE])(?!\1|\2|\3|\4|\5|\6)[CLIMATE]
ACCLIMATE ACCLIMATED ACCLIMATES ACHROMATICLENS ANASTIGMATICLENS ARISTOLOCHIACLEMATITIS ARITHMETICAL ARITHMETICALAID ARITHMETICALLY ASTRONOMICALTELESCOPE AUTOMATICELEVATOR BIOLOGICALTIME CELESTIALMECHANICS ...
I believe the problem is that you can back reference the result of a character class match, but not its terms. I'd love to be proven wrong, BTW. You can see the entire result here: