Concatenate variable and lookup expression

Using json-doc() I store JSON in a variable $db.data.work.
Then I process XML where I have placeholders like this

<persName>{API:?*[?role_en='Composer']?person?firstName} {API:?*[?role_en='Composer']?person?lastName})</persName>

The idea is to use the part after "API:" in order to retrieve the values from $db.data.work and replace the placeholders. How could I concatenate the variable and the lookup expressions in order them to be evaluated?

<template name="replace.api.placeholders">

    <variable name="pattern.api.placeholders" as="xs:string" expand-text="no">\{API:(.+?)\}</variable>
    <analyze-string select="." regex="{$pattern.api.placeholders}">
      <matching-substring>
        <value-of select="
            let $db.key := regex-group(1)
            return
              if ($db.data.work<appended-retrieved-expression>) then
                $db.data.work<appended-retrieved-expression
                else
                  ."/>
      </matching-substring>
      <non-matching-substring>
        <value-of select="."/>
      </non-matching-substring>
    </analyze-string>
  </template>

I naively tried to concatenate both as strings and then evaluate using the curly braces or without.

    <value-of select="
                let $statement :=     '$db.data.work' || '?persons?*[?role_en=\"Composer\"]?person?firstName' 
                return
                  if ({$statement}) then
                            {$statement}
                  else              
                      ."/>

Note that just injecting one key works fine:

    <value-of select="
                let $db.key := regex-group(1)
                return
                  if ($db.data.work?($db.key)) then
                    $db.data.work?($db.key)
                  else              
                      ."/>

Use xsl:evaluate:

<xsl:evaluate xpath="regex-group(1)" context-item="$db.data.work"/>

Needs Saxon 10 or higher for HE, for PE and EE 9.8 or higher.

Minimal example (your input):

<persName>{API:?*[?role_en='Composer']?person?firstName} {API:?*[?role_en='Composer']?person?lastName})</persName>

XSLT

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
	xmlns:xs="http://www.w3.org/2001/XMLSchema"
	exclude-result-prefixes="#all"
	version="3.0">

  <xsl:mode on-no-match="shallow-copy"/>

  <xsl:template match="persName">
    <xsl:variable name="pattern.api.placeholders" as="xs:string" expand-text="no">\{API:(.+?)\}</xsl:variable>
    <xsl:analyze-string select="." regex="{$pattern.api.placeholders}">
      <xsl:matching-substring>
        <xsl:evaluate xpath="regex-group(1)" context-item="$db.data.work"/>
      </xsl:matching-substring> 
    </xsl:analyze-string>
  </xsl:template>
  
  <xsl:param name="db.data.work" select="parse-json($json-string)"/>
  
  <xsl:param name="json-string" as="xs:string" expand-text="no">[
    { "role_en" : "Composer", "person": { "firstName": "Johann Sebastian", "lastName":"Bach" } }
  ]</xsl:param>
  
</xsl:stylesheet>