In the context of performing replacements in the order defined by the
descending length of the substrings matched by a set of XSLT 2.0
regexes in the input, I have been looking into two questions more
deeply:

1. Setting up a "meta-regex" to distinguish regexes whose matches may
   vary in length from those regexes with fixed-length matches. See
   thread "Meta-regex for variable-length matching regexes (XSLT)" on
   regexadvice.com: http://regexadvice.com/forums/thread/35317.aspx
   Consider especially the last posting in the thread:
http://regexadvice.com/forums/permalink/35317/35588/ShowThread.aspx#35588

2. Specifying a procedure to determine the length of the matches of a
   regex with fixed-length matches. See thread "Meta-regex to find
   fixed length of regex match (XSLT)" on regexadvice.com:
   http://regexadvice.com/forums/thread/35318.aspx
   Consider especially the last posting in the thread:
http://regexadvice.com/forums/permalink/35318/35616/ShowThread.aspx#35616

If someone is interested in these topics, I would be glad if you could
share your opinions here.

(I hope it is acceptable on the list to just point you there, because
otherwise that would be a lot of material to repeat here.)