This can be done most easily using the FXSL Library, more exactly its str-map
template.
Here is a simple example.
This transformation:
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:msxsl="urn:schemas-microsoft-com:xslt"
xmlns:testmap="testmap"
exclude-result-prefixes="xsl testmap"
>
<xsl:import href="str-map.xsl"/>
<!-- to be applied on any xml source -->
<testmap:testmap/>
<xsl:output omit-xml-declaration="yes" indent="yes"/>
<xsl:template match="/">
<xsl:variable name="vTestMap" select="document('')/*/testmap:*[1]"/>
<xsl:call-template name="str-map">
<xsl:with-param name="pFun" select="$vTestMap"/>
<xsl:with-param name="pStr" select="'abc&d f'"/>
</xsl:call-template>
</xsl:template>
<xsl:template match="testmap:*">
<xsl:param name="arg1"/>
<xsl:choose>
<xsl:when test="$arg1 = '&'">%26</xsl:when>
<xsl:when test="$arg1 = ' '">%20</xsl:when>
<xsl:otherwise><xsl:value-of select="$arg1"/></xsl:otherwise>
</xsl:choose>
</xsl:template>
</xsl:stylesheet>
when applied on any XML document (not used), produces the wanted result:
abc%26d%20f