<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision: 297028 $ -->
<refentry xml:id="function.preg-replace-callback" xmlns="http://docbook.org/ns/docbook">
 <refnamediv>
  <refname>preg_replace_callback</refname>
  <refpurpose>Perform a regular expression search and replace using a callback</refpurpose>
 </refnamediv>

 <refsect1 role="description">
  &reftitle.description;
  <methodsynopsis>
   <type>mixed</type><methodname>preg_replace_callback</methodname>
   <methodparam><type>mixed</type><parameter>pattern</parameter></methodparam>
   <methodparam><type>callback</type><parameter>callback</parameter></methodparam>
   <methodparam><type>mixed</type><parameter>subject</parameter></methodparam>
   <methodparam choice="opt"><type>int</type><parameter>limit</parameter><initializer>-1</initializer></methodparam>
   <methodparam choice="opt"><type>int</type><parameter role="reference">count</parameter></methodparam>
  </methodsynopsis>
  <para>
   The behavior of this function is almost identical to
   <function>preg_replace</function>, except for the fact that instead of
   <parameter>replacement</parameter> parameter, one should specify a
   <parameter>callback</parameter>.
  </para>
 </refsect1>

 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>pattern</parameter></term>
     <listitem>
      <para>
       The pattern to search for. It can be either a string or an array with
       strings.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>callback</parameter></term>
     <listitem>
      <para>
       A callback that will be called and passed an array of matched elements
       in the <parameter>subject</parameter> string. The callback should
       return the replacement string.
      </para>
      <para>
       You'll often need the <parameter>callback</parameter> function
       for a <function>preg_replace_callback</function> in just one place.
       In this case you can use an
       <link linkend="functions.anonymous">anonymous function</link> (since
       PHP 5.3.0) or <function>create_function</function> to
       declare an anonymous function as callback within the call to
       <function>preg_replace_callback</function>. By doing it this way
       you have all information for the call in one place and do not
       clutter the function namespace with a callback function's name
       not used anywhere else.
      </para>
      <para>
       <example>
        <title><function>preg_replace_callback</function> and 
        <function>create_function</function></title>
        <programlisting role="php">
<![CDATA[
<?php
/* a unix-style command line filter to convert uppercase
 * letters at the beginning of paragraphs to lowercase */
$fp = fopen("php://stdin", "r") or die("can't read stdin");
while (!feof($fp)) {
    $line = fgets($fp);
    $line = preg_replace_callback(
        '|<p>\s*\w|',
        create_function(
            // single quotes are essential here,
            // or alternative escape all $ as \$
            '$matches',
            'return strtolower($matches[0]);'
        ),
        $line
    );
    echo $line;
}
fclose($fp);
?>
]]>
        </programlisting>
       </example>
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>subject</parameter></term>
     <listitem>
      <para>
       The string or an array with strings to search and replace.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>limit</parameter></term>
     <listitem>
      <para>
       The maximum possible replacements for each pattern in each
       <parameter>subject</parameter> string. Defaults to
       <literal>-1</literal> (no limit).
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>count</parameter></term>
     <listitem>
      <para>
       If specified, this variable will be filled with the number of
       replacements done.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
 </refsect1>

 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   <function>preg_replace_callback</function> returns an array if the
   <parameter>subject</parameter> parameter is an array, or a string
   otherwise. On errors the return value is &null;
  </para>
  <para>
   If matches are found, the new subject will be returned, otherwise
   <parameter>subject</parameter> will be returned unchanged. 
  </para>
 </refsect1>

 <refsect1 role="changelog">
  &reftitle.changelog;
  <para>
   <informaltable>
    <tgroup cols="2">
     <thead>
      <row>
       <entry>&Version;</entry>
       <entry>&Description;</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry>5.1.0</entry>
       <entry>
        The <parameter>count</parameter> parameter was added
       </entry>
      </row>
     </tbody>
    </tgroup>
   </informaltable>
  </para>
 </refsect1>

 <refsect1 role="examples">
  &reftitle.examples;
  <para>
   <example>
    <title><function>preg_replace_callback</function> example</title>
    <programlisting role='php'>
<![CDATA[
<?php
// this text was used in 2002
// we want to get this up to date for 2003
$text = "April fools day is 04/01/2002\n";
$text.= "Last christmas was 12/24/2001\n";
// the callback function
function next_year($matches)
{
  // as usual: $matches[0] is the complete match
  // $matches[1] the match for the first subpattern
  // enclosed in '(...)' and so on
  return $matches[1].($matches[2]+1);
}
echo preg_replace_callback(
            "|(\d{2}/\d{2}/)(\d{4})|",
            "next_year",
            $text);

?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
April fools day is 04/01/2003
Last christmas was 12/24/2002
]]>
    </screen>
   </example>
  </para>
  <para>
   <example>
    <title><function>preg_replace_callback</function> using recursive structure
     to handle encapsulated BB code</title>
    <programlisting role="php">
<![CDATA[
<?php
$input = "plain [indent] deep [indent] deeper [/indent] deep [/indent] plain";

function parseTagsRecursive($input)
{

    $regex = '#\[indent]((?:[^[]|\[(?!/?indent])|(?R))+)\[/indent]#';

    if (is_array($input)) {
        $input = '<div style="margin-left: 10px">'.$input[1].'</div>';
    }

    return preg_replace_callback($regex, 'parseTagsRecursive', $input);
}

$output = parseTagsRecursive($input);

echo $output;
?>
]]>
    </programlisting>
   </example>
  </para>
 </refsect1>

 <refsect1 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member><function>preg_replace</function></member>
    <member><function>create_function</function></member>
    <member>&seealso.callback;</member>
   </simplelist>
  </para>
 </refsect1>

</refentry>

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