Techniques for transforming content like "<tag>content</tag>

I have a requirement to write XSLT transformers for XML documents, some
of whose element contents can consist of "encoded" XML, like this:

<other>
 <key>stuff</key>
 <value>&lt;userid&gt;98765&lt;/userid&gt;</value>
</other>

We need to transform this to this:

<other>
 <key>stuff_userid</key>
 <value>98765</value>
</other>

There's no avoiding that this will be messy, so I'm only aiming to clean
up the worst part of this process: how I parse the "encoded" XML.  A POC
for this is just using "substring-after" and "substring-before" to set
the pieces into variables.  Is there a better way to parse content like
this?


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