GNU bash, version 5.0.17(1)-release (x86_64-pc-linux-gnu)
Copyright (C) 2019 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later http://gnu.org/licenses/gpl.html

$> var=39Q08a
$> [[ $var =~ ^[:alnum:]*$ ]] && echo hi || echo nope
nope

$> [[ $var =~ ^[0-9a-zA-Z]*$ ]] && echo hi || echo nope
hi

I expect it to work like the old 0-9a-Z does, but it’s not.

Am I doing something wrong?