Replace an uppercase character followed by the lowercase version of itself (e.g. 'Qq') with just the lowercase character in XPath 3.0 with examples in XQuery 3.0 and XSLT 3.0

collapse.caps.3.xpath

function($input as xs:string) as xs:string {
  fold-left(
    function($s,$r){
      replace($s,upper-case($r)||$r,$r)
    },
    $input,
    ((1 to 26) ! format-integer(.,'a')))
}

example.collapse.caps.3.xqy

let
  $collapse.caps :=
    function($input as xs:string) as xs:string {
      fold-left(
        function($s,$r){
          replace($s,upper-case($r)||$r,$r)
        },
        $input,
        ((1 to 26) ! format-integer(.,'a')))
    }
return $collapse.caps('FOOBbar')

(: returns 'FOObar' :)

example.collapse.caps.3.xslt

<xsl:stylesheet
  version="3.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  exclude-result-prefixes="xs">
  
  <xsl:variable name="collapse.caps" as="function(xs:string) as xs:string" 
    select="
    function($input as xs:string) as xs:string {
      fold-left(
        function($s,$r){
        replace($s,upper-case($r)||$r,$r)
        },
        $input,
        ((1 to 26) ! format-integer(.,'a')))
    }"/>
  
  <xsl:template match="/">
    <xsl:sequence select="$collapse.caps('ZzZzZz')"/>
    <!-- returns 'zzz' -->
  </xsl:template>
  
</xsl:stylesheet>