> Comparing strings can also be time consuming for long strings, so I'd
> like an "inexpensive" solution, if there is one.
> 
> Any suggestions for finding the first difference in 2 strings (besides a
> linear march through the strings in my XSL transformation)?

Honestly, a march through the strings as you indicate above is what first
came to my mind.  I'd be curious to see what other solutions are presented!

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" 
      xmlns:xs="http://www.w3.org/2001/XMLSchema"
      xmlns:f="sample.namespace"
      exclude-result-prefixes="xs f"
      version="2.0">

  <xsl:output method="text" />
  <xsl:variable name="a" as="xs:string" select="'abcdefghijklmnopqrstuvwxyz'" />
  <xsl:variable name="b" as="xs:string" select="'abcdefghijklmnopqrstuvwxy0'" />
  <xsl:template match="/">
    <xsl:sequence select="f:mismatch($a, $b)" />
  </xsl:template>

  <xsl:function name="f:mismatch" as="xs:integer?">
    <xsl:param name="a" as="xs:string" />
    <xsl:param name="b" as="xs:string" />
    <xsl:sequence
      select="for $r in f:mismatch-position(
        string-to-codepoints($a), string-to-codepoints($b), 1)
        return if ($r eq 0) then () else $r" />
  </xsl:function>

  <xsl:function name="f:mismatch-position" as="xs:integer">
    <xsl:param name="a"   as="xs:integer*" />
    <xsl:param name="b"   as="xs:integer*" />
    <xsl:param name="pos" as="xs:integer" />
    <xsl:sequence select="
      if (empty($a) and empty($b))
        then 0
      else if ($a[1] ne $b[1])
        then $pos
      else
        f:mismatch-position(remove($a, 1), remove($b, 1), $pos + 1)" />
  </xsl:function>

</xsl:stylesheet>

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James A. Robinson                       jim.robinson@xxxxxxxxxxxx
Stanford University HighWire Press      http://highwire.stanford.edu/
+1 650 7237294 (Work)                   +1 650 7259335 (Fax)