Why does XSLT output all text by default?

Hi I had performed a transformation which drops a tag if it is null.

I wanted to check whether my transformation is working fine, so instead of checking it manually, I wrote one more XSLT code which just checks the presence of that particular tag in the OUTPUT XML, if it is null, then the second XSLT should output a text "FOUND". (I don't actually need some XML kind of output but I am just using XSLT for searching.)

When I tried with this XSL code ::

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:template match="/SiebelMessage//SuppressCalendar[.!='']">
      FOUND
  </xsl:template>
</xsl:stylesheet>

It outputs all the TEXT DATA that is present in the XML file,

to avoid that, I had to write this code::

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:template match="/SiebelMessage//SuppressCalendar[.!='']">
      FOUND
  </xsl:template>
  <xsl:template match="text()"/>
</xsl:stylesheet>

why did the former code outputs TEXT, why should I insist XSL to ignore all other text? is that the behavior of all XML parsers or only of my own (I am using msxml parser).

> why did the former code outputs TEXT, > why should I insist XSL to ignore all > other text? is that the behavior of > all XML parsers or only of my own

You are discovering one of the most fundamental XSLT features as specified in the Specification: the built-in templates of XSLT.

From the Spec:

> There is a built-in template rule to > allow recursive processing to continue > in the absence of a successful pattern > match by an explicit template rule in > the stylesheet. This template rule > applies to both element nodes and the > root node. The following shows the > equivalent of the built-in template > rule: > > > > > > There is also a built-in template rule > for each mode, which allows recursive > processing to continue in the same > mode in the absence of a successful > pattern match by an explicit template > rule in the stylesheet. This template > rule applies to both element nodes and > the root node. The following shows the > equivalent of the built-in template > rule for mode m. > > > > > > There is also a built-in template rule > for text and attribute nodes that > copies text through: > > > > > > The built-in template rule for > processing instructions and comments > is to do nothing. > > > > The built-in template rule for > namespace nodes is also to do nothing. > There is no pattern that can match a > namespace node; so, the built-in > template rule is the only template > rule that is applied for namespace > nodes. > > The built-in template rules are > treated as if they were imported > implicitly before the stylesheet and > so have lower import precedence than > all other template rules. Thus, the > author can override a built-in > template rule by including an explicit > template rule.

So, the reported behavior is the result of the application of the built-in templates -- the 1st and 2nd of all three of them.

It is a good XSLT design pattern to override the built-in templates with your own that will issue an error message whenever called so that the programmer immediately knows his transformation is "leaking":

For example, if there is this XML document:

<a>
  <b>
    <c>Don't want to see this</c>
  </b>
</a>

and it is processed with this transformation:

<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="a|b">
   <xsl:copy>
      <xsl:attribute name="name">
        <xsl:value-of select="name()"/>
      </xsl:attribute>
      <xsl:apply-templates/>
   </xsl:copy>
 </xsl:template>
</xsl:stylesheet>

the result is:

<a name="a">
   <b name="b">Don't want to see this</b>
</a>

and the programmer will be greatly confused how the unwanted text appeared.

However, just adding this catch-all template helps avoid any such confusion and catch errors immediately:

 <xsl:template match="*">
  <xsl:message terminate="no">
   WARNING: Unmatched element: <xsl:value-of select="name()"/>
  </xsl:message>
  
  <xsl:apply-templates/>
 </xsl:template>

Now, besides the confusing output the programmer gets a warning that explains the problem immediately:

 WARNING: Unmatched element: c

Later Addition by Michael Kay for XSLT 3.0

In XSLT 3.0, rather than adding a catch-all template rule, you can specify the fallback behaviour on an xsl:mode declaration. For example, <xsl:mode on-no-match="shallow-skip"/> causes all nodes that are not matched (including text nodes) to be skipped, while <xsl:mode on-no-match="fail"/> treats a no-match as an error, and <xsl:mode warning-on-no-match="true"/> results in a warning.

There are several built in template rules in XSL, one of which is this:

<xsl:template match="text()|@*">
  <xsl:value-of select="."/>
</xsl:template>

It outputs text.