xml-dev - XML Schema: regular expression challenge

["Roger L. Costello" <costello@mitre.org>]

Hi Folks,

I am trying to write an XML schema which declares an element, foo, to be
a string, 5-10 characters in length, OR empty.  For example, here are
valid instances of foo:

    <foo>HiThere</foo>
or 
    <foo></foo>

Here's my attempt at a declaration for foo:

    <element name="foo">
        <simpleType base="string">
            <pattern value="([a-z]|[A-Z]){5,10}|()"/>
        </simpleType>
    </element>

In an instance document when I set the content of foo to be empty:

    <foo></foo>

Oracle's schema processor gives me an error.  That either means that

(a) Oracle's schema processor has a bug, or
(b) My regular expression is not correct.

Let's assume that it is the later.  Any suggestions on how else to
express the regular expression?  Note that the following regular
expressions also failed:

    ([a-z]|[A-Z]){5,10}|(){0}
    ([a-z]|[A-Z]){5,10}|(.){0}

As an aside, if an element contains a string of length=0, is that
element "empty"? and thus the element can be expressed as <foo/>?

/Roger