Question

I want to change some attribute values only for some specific element.
For example, is it possible to change @lang only for the books whose price is 29.99 in below XML?

<book>
  <title lang="fr">Harry Potter</title>
  <price>29.99</price>
</book>

<book>
  <title lang="eng">Learning XML</title>
  <price>39.95</price>
</book>

Answer 1

This short and simple transformation (no explicit conditionals at all):

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

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

 <xsl:template match="book[price=29.99]/title/@lang">
  <xsl:attribute name="lang">ChangedLang</xsl:attribute>
 </xsl:template>
</xsl:stylesheet>

when applied on the provided XML (wrapped to a wellformed XML document):

<t>
    <book>
        <title lang="fr">Harry Potter</title>
        <price>29.99</price>
    </book>
    <book>
        <title lang="eng">Learning XML</title>
        <price>39.95</price>
    </book>
</t>

produces the wanted, correct result:

<t>
   <book>
      <title lang="ChangedLang">Harry Potter</title>
      <price>29.99</price>
   </book>
   <book>
      <title lang="eng">Learning XML</title>
      <price>39.95</price>
   </book>
</t>

Explanation: Overriding the identity rule for the desired attribute.