#Generating lowercase character
format "%c" [expr 97 + [rand 26]]
Then simply use one of the digits as your regular expression, and the other as the character to replace with. Use the -nocase option if you'd like the matching to be case-insensitive (will not alter the replacing character however).
To have alphanumerics you could do a rand() * 60 and check if the result is <= 10 and add 48, or is <= 36 to add 55 and else add 61. (I didn't test the exact result, the adds are +/-1 therefore ^-^)
Just as a remark for the future.