According to the BNF, "string" is yet to be defined.
For future interoperability, we will probably want to use namespaces for
qualifiers (for seasons and calendars) in later versions of EDTF. Then, we
may want to use QNames, URL, URI, IRI, LEIRI, URN ...
As of today, we want a short and easy way to express qualifiers. To avoid
ambiguity with later namespaced ones, I suggest to use xs:NCName as a
datatype for qualifiers as defined at
http://www.w3.org/TR/xml-names11/#NT-NCName for details.
For short, we can roughly simplify the description of an xs:NCName as
beginning with a letter and followed by letters and / or digits. Some
other characters are allowed, but what is important here is that colons
(:) are excluded.
This is a good example of how an xs:NCName can be used: southernHemisphere
does not contains any space or colon and southern_hemisphere would also be
a valid xs:NCName according to W3C.