<user> is a sequence of letters and digits, where '.', '-' and '_' can appear between them.

Examples of valid users: "stephan", "mike03", "s.johnson", "st_steward", "softuni-bulgaria", "12345".

Examples of invalid users: ''--123", ".....", "nakov_-", "_steve", ".info".

<host> is a sequence of at least two words, separated by dots '.'. Each word is sequence of letters and can have hyphens '-' between the letters.

Examples of hosts: "softuni.bg", "software-university.com", "intoprogramming.info", "mail.softuni.org".

Examples of invalid hosts: "helloworld", ".unknown.soft.", "invalid-host-", "invalid-".

Examples of valid emails: info@softuni-bulgaria.org, kiki@hotmail.co.uk, no-reply@github.com, s.peterson@mail.uu.net, info-bg@software-university.software.academy.

Examples of invalid emails: --123@gmail.com, …@mail.bg, .info@info.info, _steve@yahoo.cn, mike@helloworld, mike@.unknown.soft., s.johnson@invalid-.