<sxi:Rules>
    <sxi:SubStringReplace>
        <sxi:StartAt>-1</sxi:StartAt>
        <sxi:Pattern>0000000000</sxi:Pattern>
        <sxi:ReplaceWith input="yes">aNum</sxi:ReplaceWith>
    </sxi:SubStringReplace>
</sxi:Rules>

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="2.0">
    <xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>
    <xsl:strip-space elements="*"/>
    <!-- identity transform -->
    <xsl:template match="@*|node()">
        <xsl:copy>
            <xsl:apply-templates select="@*|node()"/>
        </xsl:copy>
    </xsl:template>
    
    <xsl:template match="//*[local-name()='CustomerCode']">
        <!-- Define a variable to contain the length of the input value
            The dot you see in the expression is the current element we are working on. 
            In this case CustomerCode is  the Input Value. See the match above-->
        <xsl:variable name="inputLength" select="string-length(.)" />
        <xsl:copy>
            <!-- Now we create a variable to hold our padding. 
                It is a substring of our 10 zeros less the length of our input -->
            <xsl:variable name="padding" select="substring('0000000000',1,10 - $inputLength)"/>
            
            <!-- Finally we simply concatinate our padding which is the correct length with the input value.
                Don't forget the dot you see there is the current element we matched on in this case CustomerCode Input Value -->
            <xsl:value-of select="concat($padding,.)" />
        </xsl:copy>
    </xsl:template>
    
    <xsl:template match="//*[local-name()='SiteCode']">
        <!-- Define a variable to contain the length of the input value
            The dot you see in the expression is the current element we are working on. 
            In this case SiteCode is  the Input Value. See the match above-->
        <xsl:variable name="inputLength" select="string-length(.)" />
        <xsl:copy>
            <!-- Now we create a variable to hold our padding. 
                It is a substring of our 10 zeros less the length of our input -->
            <xsl:variable name="padding" select="substring('0000000000',1,10 - $inputLength)"/>
            
            <!-- Finally we simply concatinate our padding which is the correct length with the input value.
                Don't forget the dot you see there is the current element we matched on in this case SiteCode Input Value -->
            <xsl:value-of select="concat($padding,.)" />
        </xsl:copy>
    </xsl:template>
</xsl:stylesheet>

<?xml version="1.0" encoding="UTF-8"?>
<XServiceBroker xmlns="http://www.sxi.co.za/XMLSchema">
    <CustomerCode>1222B3</CustomerCode>
    <SiteCode>ZB22B3</SiteCode>
</XServiceBroker>