hyphen](https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_282),
with the restriction that hyphen is not allowed as first character of the
user name. Interestingly no size limit is declared, i.e. in neither
- direction, meaning that strictly speaking according to POSIX both the empty
+ direction, meaning that strictly speaking, according to POSIX, both the empty
string is a valid user name as well as a string of gigabytes in length.
* Debian/Ubuntu based systems enforce the regular expression