XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (641 page)

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A
quantifier
is either one of the symbols
?
,
*
, or
+
, or a
quantity
enclosed between curly braces. A quantifier may be followed by
?
to indicate that it is a non-greedy quantifier.

A
quantity
is either a number, or a number followed by a comma, or two numbers separated by a comma:
number
here means a sequence of one or more digits.

The
piece
A?
matches a single
A
or a zero-length string;
A*
matches a sequence of zero or more
A
s, while
A+
matches a sequence of one or more
A
s.