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 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.

> 2001-21^southernHemisphere
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.