Piece

An atom, either a normal character, a character class, or a parenthesized regular expression, possibly followed by a quantifier.

piece :: = atom quantifier?

The following table shows valid pieces R and their set of strings L(R).

Valid pieces R for atoms S and non-negative integers n and m such that n<=m	Set of strings L(R)
S	All strings in L(S).
S?	The empty string, and all strings in L(S).
S*	All strings in L(S?) and all strings ST with S in L(S)* and T in L(S). All concatenations of zero or more strings from L(S).
S+	All strings ST with S in L(S) and T in L(S). All concatenations of one or more strings from L(S)*.
S{n, m}	All strings ST with S in L(S) and T in L(S{n-1, m-1}). All sequences of at least n, and at most m, strings from L(S).
S{n}	All strings in L(S{n,n}). All sequences of exactly n strings from L(S).
S{n,}	All strings in L(S{n}S). All sequences of exactly n strings from L(S)*.
S{0, m}	All strings ST with S in L(S?) and T in L(S{0, m-1}). All sequences of at most m, strings from L(S).
S{0,0}	The set containing only the empty string.