You should have a look on the -regextype argument of find, see manpage:
-regextype type
Changes the regular expression syntax understood by -regex and -iregex
tests which occur later on the command line. Currently-implemented
types are emacs (this is the default), posix-awk, posix-basic,
posix-egrep and posix-extended.
I guess the emacs type doesn’t support the [[:digit:]] construct. I tried it with posix-extended and it worked as expected:
find -regextype posix-extended -regex '.*[1234567890]'
find -regextype posix-extended -regex '.*[[:digit:]]'