a 0 1 2 3 \n
. contains all characters but \n.
(union) is the regular expression, that matches all input that is matched by a or by b.
(concatenation) is the regular expression, that matches the input matched by a followed by the input matched by b.
(kleene closure) matches zero or more repetitions of the input matched by a
is equivalent to n times the concatenation of a. So a{4} for instance is equivalent to the expression a a a a. The decimal integer n must be positive.
is equivalent to at least n times and at most m times the concatenation of a. So a{2,4} for instance is equivalent to the expression a a a? a?. Both n and m are non negative decimal integers and m must not be smaller than n.