> Just to check that by "valid potential QName" you mean a string that is in
> the lexical space of xs:QName and will therefore constitute a valid QName
> provided the prefix is in scope?
>
> ($x castable as xs:QName) will not work because (a) it also tests whether
> the prefix is in scope, and (b) to avoid the issues of it therefore being
> context-sensitive, the W3C spec says this is only allowed if the argument
> ($x) is a string literal.
>
> So you do need to use the regex approach.

Could you do a  fn:prefix-from-QName($x) = fn:in-scope-prefixes(.)
check before doing the castable as check?

  http://www.w3.org/TR/2007/REC-xpath20-20070123/#id-castable
  "[if] the input argument of the expression is of type xs:string but it is
   not a literal string, the result of the castable expression is false."

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