And, of course, the second function would just move the character
class into the non-greedy search:

B replace('THEN THE CURTAIN','(^.*?[AEIOU]).*$','$1')

... returns "THE".

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

B  <xsl:template match="/" name="xsl:initial-template">
B B B  <xsl:iterate select="(1 to count(analyze-string($input,
'[AEIOU]')/*:match))">
B B B B B  <xsl:param name="string" as="xs:string" select="$input"/>
B B B B B  <xsl:sequence select="$string"/>
B B B B B  <xsl:variable name="match"
select="replace($string,'(^.*?[AEIOU]).*$','$1')"/>
B B B B B  <xsl:next-iteration>
B B B B B B B  <xsl:with-param name="string"
select="substring-after($string,
$match)"/>
B B B B B  </xsl:next-iteration>
B B B  </xsl:iterate>
B  </xsl:template>

THEN THE CURTAIN FELL
N THE CURTAIN FELL
B CURTAIN FELL
RTAIN FELL
IN FELL
N FELL