What is the extended regular expression for following question.

E.g. Social security number in the format of 999-99-9999.

Answer: [0-9]{3}-[0-9]{2}-[0-9]{4}

1) Identifiers which can be between 1 and 10 character long, must starting with a letter or an underscore. The following characters can be letters or underscores or digits. (e.g. number, _name1, isOK).