The general solution in DFDL is to use the combination of an optional repeating element inside a hidden group.

You need to be careful that this optional hidden element does not consume the next piece of wanted data by mistake. If all the unwanted elements have known initiators then you are ok. If you don't know the initiators, but know what is coming next, then one approach is as follows:

<xs:complexType>
        <xs:sequence>
                <xs:element name="From" type="NameType" dfdl:initiator="From:%WSP*;" terminator="%NL;%WSP*;" />
                <xs:element name="To" type="NameType" dfdl:initiator="To:%WSP*;" terminator="%NL;%WSP*;"/>
                <xs:sequence dfdl:hiddenGroupRef="UnwantedGroup" />
                <xs:element name="Subject" type="xs:string" dfdl:initiator="Subject:%WSP*;" terminator="%NL;%WSP*;"/>
        </xs:sequence>
</xs:complexType>

<xs:group name="UnwantedGroup>
        <xs:sequence>
                <xs:element name="UnwantedHeaders" maxOccurs="unbounded" />
                        <xs:complexType>
                                <xs:sequence>
                                        <xs:element name="Unwanted" type="xs:string" terminator="%NL;%WSP*;">
                                                  <xsd:annotation><xsd:appinfo source="http://www.ogf.org/dfdl/">
                                                         <dfdl:discriminator test="{fn:not(fn:startWith("Subject:"))}"/>
                                                 </xsd:appinfo></xsd:annotation>
                                        </xs:element>
                                </xs:sequence>
                        </xs:complexType>
                </xs:element>
        </xs:sequence>
</xs:complexType>

The hidden loop should consume all header lines that do not start with "Subject:" and stop when it reaches one that does.

I've used a terminator for the header lines, you may have used a separator with separatorPolicy 'suppressed'. Either should work, but the terminator gives you the opportunity to handle data where the final CRLF is missing (via property dfdl:documentFinalTerminatorCanBeMissing).

Regards

Steve Hanson
Architect, Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848



From:        "Garriss Jr., James P." <jgarriss@mitre.org>
To:        "dfdl-wg@ogf.org" <dfdl-wg@ogf.org>,
Date:        01/03/2013 13:15
Subject:        [DFDL-WG] Can I ignore data I don't want in DFDL?
Sent by:        dfdl-wg-bounces@ogf.org

---

Suppose I am using DFDL to parse email headers.  Suppose the RFC only allows 3 headers:  To, From, Subject.  DFDL can handle this, no problem.
 
But suppose I get an email that includes a 4th header, one I have not planned for (i.e., have not included in the DFDL schema), don’t care about, and don’t want in the infoset.  Like so:
 
From: <john@doe.com>
To: <jane@doe.com>
Keywords:  sales                       <-- this line should be ignored!
Subject:  Latest sales figures
 
Can DFDL handle this?  Does it have a mechanism for allowing me to ignore (and thus drop) data I haven’t planned for and don’t care about?--
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