All variables start with a single dollar-sign ("$") followed by a letter ("A"..."Z", "a"..."z"). Further characters may also contain digits ("0"..."9") or underscores ("_").
The length of a variable-name is (in fact) not limited and all characters are significant, but upper- and lowercase characters are treated the same, so $ABC is exactly the same variable as $abc.
Examples for valid variables:
$a $i________ $a_b $The_answer_is_REALLY_42
Examples for invalid variables:
a$
^^
$____i____
^
$a.b
^
$The_answer_is_REALLY_42!!!
^^^
$1st_cut
^