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add information about the == operator, && / || priorities and

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expression optimization/debugging


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@101124 13f79535-47bb-0310-9956-ffa450edef68
This commit is contained in:
André Malo
2003-08-28 15:37:17 +00:00
parent a770ae9e77
commit b1169b4e24
2 changed files with 68 additions and 8 deletions
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38
docs/manual/mod/mod_include.html.en
View File

@@ -507,6 +507,7 @@
<dd>true if <var>string</var> is not empty</dd>
<dt><code><var>string1</var> = <var>string2</var><br />
<var>string1</var> == <var>string2</var><br />
<var>string1</var> != <var>string2</var></code></dt>
<dd><p>Compare <var>string1</var> with <var>string2</var>. If
@@ -514,11 +515,12 @@
then it is treated as a regular expression. Regular expressions are
implemented by the <a href="http://www.pcre.org">PCRE</a> engine and
have the same syntax as those in <a href="http://www.perl.com">perl
5</a>.</p>
5</a>. Note that <code>==</code> is just an alias for <code>=</code>
and behaves exactly the same way.</p>
<p>If you are matching positive (<code>=</code>), you can capture
grouped parts of the regular expression. The captured parts are
stored in the special variables <code>$1</code> ..
<p>If you are matching positive (<code>=</code> or <code>==</code>), you
can capture grouped parts of the regular expression. The captured parts
are stored in the special variables <code>$1</code> ..
<code>$9</code>.</p>
<div class="example"><h3>Example</h3><p><code>
@@ -566,6 +568,10 @@
&lt;!--#if expr="($a = test1) &amp;&amp; ($b = test2)" --&gt;
</code></p></div>
<p>The boolean operators <code>&amp;&amp;</code> and <code>||</code>
share the same priority. So if you want to bind such an operator more
tightly, you should use parentheses.</p>
<p>Anything that's not recognized as a variable or an operator
is treated as a string. Strings can also be quoted:
<code>'string'</code>. Unquoted strings can't contain whitespace
@@ -578,6 +584,30 @@
and<br />
<br />
<code>'<var>string1</var>&nbsp;&nbsp;&nbsp;&nbsp;<var>string2</var>'</code> results in <code><var>string1</var>&nbsp;&nbsp;&nbsp;&nbsp;<var>string2</var></code>.</p></div>
<div class="note"><h3>Optimization of Boolean Expressions</h3>
<p>If the expressions become more complex and slow down processing
significantly, you can try to optimize them according to the
evaluation rules:</p>
<ul>
<li>Expressions are evaluated from left to right</li>
<li>Binary boolean operators (<code>&amp;&amp;</code> and <code>||</code>)
are short circuited whereever possible. In conclusion with the rule
above that means, <code class="module"><a href="../mod/mod_include.html">mod_include</a></code> evaluates at first
the left expression. If the left result if sufficient to determine
the end result, processing stops here. Otherwise it evaluates the
right side and computes the end result from both left and right
results.</li>
<li>Short circuit evaluation is turned off as long as there are regular
expressions to deal with. These must be evaluated to fill in the
backreference variables (<code>$1</code> .. <code>$9</code>).</li>
</ul>
<p>If you want to look how a particular expression is handled, you can
recompile <code class="module"><a href="../mod/mod_include.html">mod_include</a></code> using the
<code>-DDEBUG_INCLUDE</code> compiler option. This inserts for every
parsed expression tokenizer information, the parse tree and how it is
evaluated into the output sent to the client.</p>
</div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="SSIEndTag" id="SSIEndTag">SSIEndTag</a> <a name="ssiendtag" id="ssiendtag">Directive</a></h2>

38
docs/manual/mod/mod_include.xml
View File

@@ -474,6 +474,7 @@
<dd>true if <var>string</var> is not empty</dd>
<dt><code><var>string1</var> = <var>string2</var><br />
<var>string1</var> == <var>string2</var><br />
<var>string1</var> != <var>string2</var></code></dt>
<dd><p>Compare <var>string1</var> with <var>string2</var>. If
@@ -481,11 +482,12 @@
then it is treated as a regular expression. Regular expressions are
implemented by the <a href="http://www.pcre.org">PCRE</a> engine and
have the same syntax as those in <a href="http://www.perl.com">perl
5</a>.</p>
5</a>. Note that <code>==</code> is just an alias for <code>=</code>
and behaves exactly the same way.</p>
<p>If you are matching positive (<code>=</code>), you can capture
grouped parts of the regular expression. The captured parts are
stored in the special variables <code>$1</code> ..
<p>If you are matching positive (<code>=</code> or <code>==</code>), you
can capture grouped parts of the regular expression. The captured parts
are stored in the special variables <code>$1</code> ..
<code>$9</code>.</p>
<example><title>Example</title>
@@ -533,6 +535,10 @@
&lt;!--#if expr="($a = test1) &amp;&amp; ($b = test2)" --&gt;
</example>
<p>The boolean operators <code>&amp;&amp;</code> and <code>||</code>
share the same priority. So if you want to bind such an operator more
tightly, you should use parentheses.</p>
<p>Anything that's not recognized as a variable or an operator
is treated as a string. Strings can also be quoted:
<code>'string'</code>. Unquoted strings can't contain whitespace
@@ -551,6 +557,30 @@
>string2</var>'</code> results in <code><var
>string1</var>&nbsp;&nbsp;&nbsp;&nbsp;<var>string2</var></code>.</p>
</example>
<note><title>Optimization of Boolean Expressions</title>
<p>If the expressions become more complex and slow down processing
significantly, you can try to optimize them according to the
evaluation rules:</p>
<ul>
<li>Expressions are evaluated from left to right</li>
<li>Binary boolean operators (<code>&amp;&amp;</code> and <code>||</code>)
are short circuited whereever possible. In conclusion with the rule
above that means, <module>mod_include</module> evaluates at first
the left expression. If the left result if sufficient to determine
the end result, processing stops here. Otherwise it evaluates the
right side and computes the end result from both left and right
results.</li>
<li>Short circuit evaluation is turned off as long as there are regular
expressions to deal with. These must be evaluated to fill in the
backreference variables (<code>$1</code> .. <code>$9</code>).</li>
</ul>
<p>If you want to look how a particular expression is handled, you can
recompile <module>mod_include</module> using the
<code>-DDEBUG_INCLUDE</code> compiler option. This inserts for every
parsed expression tokenizer information, the parse tree and how it is
evaluated into the output sent to the client.</p>
</note>
</section>
<directivesynopsis>