Remove all characters except letters, digits and !#$%&'*+-=?^_`{|}~@.[].
var_dump(filter_var('john@example.com', FILTER_SANITIZE_EMAIL));
var_dump(filter_var("!#$%&'*+-=?^_`{|}~.[]@example.com", FILTER_SANITIZE_EMAIL));
var_dump(filter_var('john/@example.com', FILTER_SANITIZE_EMAIL));
var_dump(filter_var('john\@example.com', FILTER_SANITIZE_EMAIL));
var_dump(filter_var('joh n@example.com', FILTER_SANITIZE_EMAIL));
Results:
string(16) "john@example.com"
string(33) "!#$%&'*+-=?^_`{|}~.[]@example.com"
string(16) "john@example.com"
string(16) "john@example.com"
string(16) "john@example.com"