Easy programming problems

An identifier in Java, C++ and most other programming languages must begin with a letter and then may be followed by any number of letters or digits.

It is possible that underscores (_) will also appear, but only in the middle and never two consecutively.

Write a program to read a string and output whether it is a valid or invalid identifier. Each string will be 10 characters or less in size.

Example 1:
Enter id: UAB_HSPC
Answer: UAB_HSPC is a valid identifier

Example 2:
Enter id: _a_b__2
Answer: _a_b__2 is not a valid identifier


My ruby solution (which is not complete):

print "Identifier: "

identifier = Array.new
   
    def words(identifier)
        identifier.each_char.to_a {|word| words}
    end

        if words(gets).first.match /^[a-zA-Z]+$/i
            print "The identifier is valid."
        else
            print "The identifier is not valid."
        end