[xml] Lib xml : relax NG : String pattern not match !

["Jean-Francois Dupont-Viel (QC/EMC)" <jean-francois dupont-viel ericsson com>]

Hi,

I have a xml file that I am validating with a relax NG file.

One of the check is this :

<element name="clf.sbc.default.location">
        <data type="string">                                    
                <param
name="pattern">^(xDSL;[a-zA-Z0-9]+;[0-9]+;[a-zA-Z0-9]+;[a-zA-Z0-9]+;[a-z
A-Z0-9]+)?$</param>
        </data>                                 
</element>      

Which shall valid either the following string :
xDSL;aaa;111;aaa;bbb;ccc
 or
<empty string>

But it doesn't get the empty string.  All other RegExp applications does
accept it.

Is this a bug in libxml ?

Thanks

Jean-Francois