<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: [email protected], [email protected], [email protected], [email protected], [email protected].

Examples of invalid emails: [email protected], …@mail.bg, [email protected], [email protected], mike@helloworld, [email protected]., s.johnson@invalid-.