RelaxNG choice and regular expression - relaxng

Question

I have a RelaxNG based XML instance which should allow (choice) align="right", align="left" and align="{ anything here }". The right/left is easy:
attribute name="align">
<a:documentation>doc for attrib</a:documentation>
<choice>
<value>left</value>
<a:documentation>doc for left</a:documentation>
<value>right</value>
<a:documentation>doc for right</a:documentation>
</choice>
</attribute>
But how can I allow the user to enter { ... } as an alternative to the two values above?

Answer 1

You need to use the XML Schema string type and set a pattern on it. Here's an example:
<grammar xmlns="http://relaxng.org/ns/structure/1.0"
datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes">
<start>
<element name="top">
<oneOrMore>
<element name="foo">
<attribute name="align">
<choice>
<value>left</value>
<value>right</value>
<data type="string">
<param name="pattern">\{.+\}</param>
</data>
</choice>
</attribute>
</element>
</oneOrMore>
</element>
</start>
</grammar>
And a test file:
<top>
<!-- valid cases -->
<foo align="{fasf}"></foo>
<foo align="left"></foo>
<foo align="right"></foo>
<!-- invalid cases -->
<foo align="fasf"></foo>
<foo align="{}"></foo>
<foo align="a{c}b"></foo>
</top>
Note the comments that delimit valid and invalid cases.
The regular expression I used allows things like align="{ab{c}d}". Curly braces are valid inside the curly braces that start and end the attribute value. I've not see any reason to disallow it.