How to extract unique characters from a string using XSLT 1.0?

Question

one of the toughest challenges I have ever faced in XSLT designing ..

How to copy the unique characters in a given string ..
Test xml is:

<root>
<string>aaeerstrst11232434</string>
</root>

The output I am expecting is:

<string>aerst1234</string>

Answer 1

Here is an XSLT 1.0 solution, shorter than the currently selected answer and easier to write as it uses the str-foldl template of FXSL.

<xsl:stylesheet version="1.0"
 xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
 xmlns:f="http://fxsl.sf.net/"
 exclude-result-prefixes="f">

 <xsl:import href="str-foldl.xsl"/>
 <xsl:output method="text"/>

 <f:addUnique/>

 <xsl:variable name="vFunAddunique" select=
  "document('')/*/f:addUnique[1]
  "/>

    <xsl:template match="string">
      <xsl:call-template name="str-foldl">
        <xsl:with-param name="pFunc" select="$vFunAddunique"/>
        <xsl:with-param name="pA0" select="''"/>
        <xsl:with-param name="pStr" select="."/>
      </xsl:call-template>
    </xsl:template>

    <xsl:template match="f:addUnique" mode="f:FXSL">
      <xsl:param name="arg1"/>
      <xsl:param name="arg2"/>

      <xsl:value-of select="$arg1"/>
      <xsl:if test="not(contains($arg1, $arg2))">
       <xsl:value-of select="$arg2"/>
      </xsl:if>
    </xsl:template>
</xsl:stylesheet>

When the above transformation is applied to the originally provided source XML document:

<root>
    <string>aaeerstrst11232434</string>
</root>

the wanted result is produced:

aerst1234

Read more about FXSL 1.x (for XSLT 1.0) here, and about FXSL 2.x (for XSLT 2.0) here.