> http://www.w3.org/TR/xslt20/#format-date defines
>   format-dateTime( $value    as xs:dateTime?,
>                    $picture  as xs:string,
> 		     $language as xs:string?,
> 		     $calendar as xs:string?,
> 		     $country  as xs:string?) as xs:string?
> The picture string is fairly regular.
> Has anyone produced a relaxng expression which captures it please?

I hadn't, but just took a crack at it. I think

 xsd:token { pattern =
 "(([^\[\]]|\[\[|\]\])+|\[[YMDdFWwHhPmsfZzCE](((\p{Nd}|\p{Nl}|\p{No}|\p{Lu}|\p{Ll}|\p{Lt}|\p{Lm}|\p{Lo})+|N|n|Nn)[to]?)?(,([0-9]+|\*)(-[0-9]+|\*)?)?\])+"
 }

does the job, (Note xsd:token, not xsd:string, as I wrote this
presuming the string to be matched has already been run through
normalize-space().) Not very well tested. See
http://dev.stg.brown.edu/staff/Syd_Bauman/temp/format-dateTime-picture-validate.tgz.