[xsl] parsing parens in the park

[Syd Bauman <Syd_Bauman@xxxxxxxxx>]

In an XSLT 2.0 stylesheet I have a string (call it $park) that is of
the format
       name1 (value1) name2 (value2) name3 (value3) ...
where the whitespace is optional, but the parentheses are not. Every
name is guaranteed to be an unqualified XML Name. Every name is
guaranteed to have an associated value, which could be a string of
most anything, including the null string or one that contains
*matched pairs of parentheses*. (A previous validation stage has, in
theory, guaranteed that there are no unmatched single parentheses in
a value.)

I need to produce a copy of this string with certain adjustments,
e.g., dropping any name-value pair where the name is a particular
string, or selecting only name-value pairs for which the value
matches a given regexp (which may include the local name of the
context node).

So it seems like a good first step would be to parse the string into
what Perl calls a hash table, or lisp calls an alist. I think that
means what I wish I could do is generate a key ala the following
(presuming that my get functions return a sequence).
  <xsl:key name="park-alist"
           match="syd:get-values( $park )"
           use="syd:get-names( $park )"/>
If I understand correctly, this isn't possible because you can't call
a function from the match= attribute, as it is a pattern, not an
expression.

So I suppose I can do this without a key by just indexing into two
separate sequences. E.g., something like
   <xsl:for-each select="syd:get-names($park)">
     <xsl:variable name="i" select="position()"/>
     <xsl:message>
       name # <xsl:value-of select="$i"/> = <xsl:value-of select="."/>
       value # <xsl:value-of select="$i"/> = <xsl:value-of select="syd:get-values($park)[$i]"/>
     </xsl:message>
   </xsl:for-each>

However, while I'm confident I could tackle the parsing those two
functions have to do in lisp or Perl, where I could scan the value
and count open- and close-parens as I go, I have no idea how to parse
them in XPath 2.0. I think maybe I could come up with a complex set
of recursive parsing templates that used substring-before() and
substring-after(), but
a) I'm not sure, and
b) there's gotta be a better way.

So, any thoughts on how to parse that string, or a better method of
approaching the whole problem, are appreciated.