How to check element existence and select substring of element value

I have an XML file :

<?xml version="1.0" encoding="windows-1250"?>
<CONTACTS>
    <CONTACT>
        <FirstName>AfgZohal</FirstName>
        <LastName>Zohal Afg</LastName>
    </CONTACT>
    <CONTACT>
        <FirstName>Arun_niit</FirstName>
        <LastName>Arun_niit</LastName>
        <EMail>nura_ice@yahoo.co.in</EMail>
    </CONTACT>
    <CONTACT>
        <FirstName>Bống M&#xc5;©nHá&#xba;&#xa3;i</FirstName>
        <LastName>Há&#xba;&#xa3;i Anh Bống M&#xc5;©n</LastName>
        <URL>http://www.facebook.com/profile.php?id=100000689849077</URL>
    </CONTACT>
</CONTACTS>

I want to add an element ID before FirstName in my xml file;I would like to extract ID from URL tag if URL is available or I want to extract the first six letters from Email address to put it in the ID(unique). Because in some contacts, there is no URL. I using XSLT for this. In my XSl file i tried in this way

   <ID>
     <xsl:value-of select="CONTACT/URL[//http='@id']"/>
     </ID>

but it's not working, this is my XSL file:

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    <xsl:output method="xml" indent="yes"/>
    <xsl:strip-space elements="*"/>

    <xsl:template match="node()|@*">
        <xsl:copy>
            <xsl:apply-templates select="node()|@*"/>
        </xsl:copy>
    </xsl:template>

    <xsl:template match="CONTACT">
        <xsl:copy>
         <ID>
         <xsl:value-of select="CONTACT/URL[//http='@id']"/>
         </ID>
              <xsl:copy-of select="FirstName|LastName|URL"/>
            <EMAILS>
                <xsl:apply-templates select="EMail"/>
            </EMAILS>
        </xsl:copy>
    </xsl:template>

    <xsl:template match="EMail">
        <EMail>
            <Type><xsl:value-of select="substring-before(
                    substring-after(.,'@'),
                    '.')"/>
            </Type>
            <Value><xsl:value-of select="."/></Value>
        </EMail>
    </xsl:template>

</xsl:stylesheet>

This is my output xml file:

<?xml version="1.0" encoding="UTF-8"?>
<CONTACTS>
<CONTACT>
<ID/>
<FirstName>AfgZohal</FirstName>
<LastName>Zohal Afg</LastName>
<EMAILS/>
</CONTACT>
<CONTACT>
<ID/>
<FirstName>Arun_niit</FirstName>
<LastName>Arun_niit</LastName>
<EMAILS>
<EMail>
<Type>yahoo</Type>
<Value>nura_ice@yahoo.co.in</Value>
<开发者_Go百科;/EMail>
</EMAILS>
</CONTACT>
<CONTACT>
<ID/>
<FirstName>Bống MũnHải</FirstName>
<LastName>Hải Anh Bống Mũn</LastName>
<URL>http://www.facebook.com/profile.php?id=100000689849077</URL>
<EMAILS/>
</CONTACT>
<CONTACT>
</CONTACTS>

This is part of my yesterday problem Novice transformation using apply-templates and string manipulation in the child node; since it's a different issue i'm raising a different question.

It seems that you want to select the 'id' part of the url string inside the URL element. You should select the substring after ?id=:

 <ID>
 <xsl:value-of select="substring-after(URL,'?id=')"/>
 </ID>

Moreover, in the template you are in the context of CONTACT, ergo to select a child of it you need just to specify the name of the element. Example:

 <xsl:template match="CONTACT">
         <xsl:value-of select="URL"/>
  </xsl:template>

Will return the value of URL while:

 <xsl:template match="CONTACT">
         <xsl:value-of select="CONTACT/URL"/>
  </xsl:template>

won't return anything, not having CONTACT any child of type CONTACT/URL.

Bonus answer to comment question:

I would like to extract ID from URL tag if URL is available or I want to extract the first six letters from Email address to put it in the ID(unique) (...) If we have one/multiple email address then we can select any one of the first six letters from the email address. I thing the contacts must have at least one email address if there is no URL...

        <ID>
            <xsl:choose>
                <xsl:when test="URL">
                    <xsl:value-of select="substring-after(URL,'?id=')"/>
                </xsl:when>
                <xsl:otherwise>
                    <xsl:value-of select="substring-before(EMail[1],'@')"/>
                </xsl:otherwise>
            </xsl:choose>
        </ID>