Documentation claims that

POSIX extended regular expressions are supported in Zabbix.

However, there is no limitation in POSIX regarding '@' symbol. In Zabbix it is used to reference global regular expressions and therefore can't be used as the first symbol in ordinary regular expressions.

This limitation should be documented (or fixed if it's considered a bug).

Also, link to Wikipedia needs to be updated. It does not point to the right article subsection.