PDA and CFG of language of regular expressions
Hint:  Let us assume an alphabet $\Sigma$. Now, the following are regular expressions (at least according to some textbooks):


$a$ is a regular expression, if $a \in \Sigma$
$\emptyset$ is a regular expression
$r_1 \cup r_2$ is a regular expression, if $r_1$ and $r_2$ are regular expressions
$r_1 \circ r_2$ is a regular expression, if $r_1$ and $r_2$ are regular expressions
Nothing else is a regular expression.


How can you summarize these formation rules as a collection of context-free production rules where $r$ is a nonterminal?