Parsing optional line prefix

[Larry Barber]

I'm parsing at text file which allows an optional ':' at the beginning of lines.
The following code works at capturing this character:
                                                    <!-- optional prefix ":" -->
                                                    <xs:element name="prefix" 
type="xs:string" maxOccurs="1" minOccurs="0"
                                                                
dfdl:lengthKind="pattern" dfdl:lengthPattern=":" dfdl:encoding="ISO-8859-1" >
                                                        <xs:annotation>
                                                            <xs:appinfo 
source=http://www.ogf.org/dfdl/>
                                                                
<dfdl:discriminator test="{ (dfdl:valueLength(., 'bytes') eq 1) }" />
                                                            </xs:appinfo>
                                                        </xs:annotation>
                                                    </xs:element>
I wanted to avoid using REGEX and changed the code as follows:
                                                    <!-- optional prefix ":" -->
                                                    <xs:element name="prefix" 
type="xs:string" maxOccurs="1" minOccurs="0"
                                                                
dfdl:lengthKind="explicit" dfdl:length="1" >
                                                        <xs:annotation>
                                                            <xs:appinfo 
source=http://www.ogf.org/dfdl/>
                                                                
<dfdl:discriminator test="{ xs:string(.) eq ':' }" />
                                                            </xs:appinfo>
                                                        </xs:annotation>
                                                    </xs:element>
I thought that this was working initially, but now it seems to just consume the 
':' character, but not put it into the infoset.
Not sure what's happening. Guess I'll just stick with the REGEX - it's only one 
character....