The full form is: <a>*<b>element
where <a> and <b> are optional decimal values,
indicating at least
<a> and at most <b> occurrences of element.
Default values are 0 and infinity so that:
A rule of the form:
<n>element is equivalent to:
<n>*<n>element
That is, exactly <n> occurrences
of <element>.
Thus 2DIGIT is a
2-digit number, and 3ALPHA is a string of three alphabetic
characters.