Evaluating last 3 or 4 characters of URL?

I am trying to evaluate the last 3 or 4 characters of a URL
so that I can add javascript in the href attribute.

It is not liking the {@url}.  Is this not just the string value
of the url attribute? Help on the syntax appreciated.

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        <xsl:attribute name="id">
          <xsl:value-of select="@id"/>
        </xsl:attribute>
      </xsl:if>
      <xsl:attribute name="href">
		<xsl:choose>
			<xsl:when test="substring({@url}, -4, 4)!='.html'">
			  javascript: do_windowopen('
			    <xsl:value-of select="@url"/>	
		          ')
			</xsl:when>
			<xsl:when test="substring({@url}, -3, 3)!='.htm'">
			  javascript: do_windowopen('
				<xsl:value-of select="@url"/>	
			  ')
			</xsl:when>					
            <xsl:otherwise>
              <xsl:value-of select="@url"/>
            </xsl:otherwise>
         </xsl:choose>
      </xsl:attribute>
...more