+ *
+ * A substring can be selected by a start index and a length or
+ * by matching a regular expression.
+ *
+ * Selecting a substring by regular expression, combined with
+ * the lvalue semantics of the substring class is a particularly
+ * powerful concept. Possible uses are for example:
+ *
+ * (assuming String S, M; regex R;)
+ * M = S(R); -> Returns matching part of S into M.
+ * S(R) = "replacement"; -> replace matching part of S.
+ * S(R) = ""; -> Removes matching part from S.
+ * S(R) == S; -> true if and only if all of S matches R exactly.