How many substrings of different lengths (non-zero) can be formed from a character string of length $$n$$ ?

$$\sum { = \left\{ {a,b} \right\},\,\,} $$ which one of the following sets is not countable.

Let $$L \subseteq \sum {^{^ * }\,} $$ where $$\,\sum { = \,\,\left\{ {a,b} \right\}\,\,} $$ which of the following is true?

Which two of the following four regular expressions are equivalent?
(i) $${\left( {00} \right)^ * }\left( {\varepsilon + 0} \right)$$
(ii) $${\left( {00} \right)^ * }$$
(iii) $${0^ * }$$
(iv) $$0\,\,{\left( {00} \right)^ * }$$