stinfo/systemd-devel
Documentation: https://systemd.io/USER_NAMES

The user/group name @USER_GROUP_NAME@ has been specified, which is accepted
according the relaxed user/group name rules, but does not qualify under the
strict rules.

The strict user/group name rules written as regular expression are:

^[a-zA-Z_][a-zA-Z0-9_-]{0,30}$

The relaxed user/group name rules accept all names, except for the empty
string; names containing NUL bytes, control characters, colon or slash
characters; names not valid UTF-8; names with leading or trailing whitespace;
the strings "." or ".."; fully numeric strings, or strings beginning in a
hyphen and otherwise fully numeric.