The XPath/XQuery grammar allows numeric literals to have a leading or trailing decimal point, for example

2. + .5

is a valid query that returns 2.5

XPath/XQuery defines the semantics of a numeric literal by reference to the rules for casting from string to number.

The rules for casting from string to number are defined in terms of the XSD-defined mapping from lexical space to value space.

But in XSD, the lexical space for xs:decimal, xs:double, and xs:float does not allow a leading or trailing decimal point.

For example, XPath defines decimal-literal with the pattern

("." [0-9]+) | ([0-9]+ "." [0-9]*)

while XSD 1.1 defines the lexical space of xs:decimal as

(\+|-)?([0-9]+(\.[0-9]*)?|\.[0-9]+)