The description of the regular expressions, which are used for
the search through the lists of the host names within this system,
may be found on the site of the FreeBSD operating system.

https://man.freebsd.org/cgi/man.cgi?re_format(7)

Here are several simple examples.

\.org$

^[-0-9a-z]*\.[-0-9a-z]*$

^[-0-9a-z]*marketing[-0-9a-z]*\.[-0-9a-z]*$

\.xn--[-0-9a-z]*$