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<title>Perform a regular expression search and replace using a callback</title>
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<h1 class="refname">preg_replace_callback</h1>
<p class="verinfo">(PHP 4 >= 4.0.5, PHP 5)</p><p class="refpurpose"><span class="refname">preg_replace_callback</span> — <span class="dc-title">Perform a regular expression search and replace using a callback</span></p>
</div>
<div class="refsect1 description" id="refsect1-function.preg-replace-callback-description">
<h3 class="title">Description</h3>
<div class="methodsynopsis dc-description">
<span class="type"><a href="language.pseudo-types.html#language.types.mixed" class="type mixed">mixed</a></span> <span class="methodname"><strong>preg_replace_callback</strong></span>
( <span class="methodparam"><span class="type"><a href="language.pseudo-types.html#language.types.mixed" class="type mixed">mixed</a></span> <code class="parameter">$pattern</code></span>
, <span class="methodparam"><span class="type"><a href="language.types.callable.html" class="type callable">callable</a></span> <code class="parameter">$callback</code></span>
, <span class="methodparam"><span class="type"><a href="language.pseudo-types.html#language.types.mixed" class="type mixed">mixed</a></span> <code class="parameter">$subject</code></span>
[, <span class="methodparam"><span class="type">int</span> <code class="parameter">$limit</code><span class="initializer"> = -1</span></span>
[, <span class="methodparam"><span class="type">int</span> <code class="parameter reference">&$count</code></span>
]] )</div>
<p class="para rdfs-comment">
The behavior of this function is almost identical to
<span class="function"><a href="function.preg-replace.html" class="function">preg_replace()</a></span>, except for the fact that instead of
<em><code class="parameter">replacement</code></em> parameter, one should specify a
<em><code class="parameter">callback</code></em>.
</p>
</div>
<div class="refsect1 parameters" id="refsect1-function.preg-replace-callback-parameters">
<h3 class="title">Parameters</h3>
<p class="para">
<dl>
<dt>
<span class="term"><em><code class="parameter">pattern</code></em></span>
<dd>
<p class="para">
The pattern to search for. It can be either a string or an array with
strings.
</p>
</dd>
</dt>
<dt>
<span class="term"><em><code class="parameter">callback</code></em></span>
<dd>
<p class="para">
A callback that will be called and passed an array of matched elements
in the <em><code class="parameter">subject</code></em> string. The callback should
return the replacement string. This is the callback signature:
</p>
<p class="para">
<div class="methodsynopsis dc-description">
<span class="type">string</span> <span class="methodname"><span class="replaceable">handler</span></span>
( <span class="methodparam"><span class="type">array</span> <code class="parameter">$matches</code></span>
)</div>
</p>
<p class="para">
You'll often need the <em><code class="parameter">callback</code></em> function
for a <span class="function"><strong>preg_replace_callback()</strong></span> in just one place.
In this case you can use an
<a href="functions.anonymous.html" class="link">anonymous function</a> to
declare the callback within the call to
<span class="function"><strong>preg_replace_callback()</strong></span>. 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.
</p>
<p class="para">
<div class="example" id="example-4794">
<p><strong>Example #1 <span class="function"><strong>preg_replace_callback()</strong></span> and
anonymous function</strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #FF8000">/* a unix-style command line filter to convert uppercase<br /> * letters at the beginning of paragraphs to lowercase */<br /></span><span style="color: #0000BB">$fp </span><span style="color: #007700">= </span><span style="color: #0000BB">fopen</span><span style="color: #007700">(</span><span style="color: #DD0000">"php://stdin"</span><span style="color: #007700">, </span><span style="color: #DD0000">"r"</span><span style="color: #007700">) or die(</span><span style="color: #DD0000">"can't read stdin"</span><span style="color: #007700">);<br />while (!</span><span style="color: #0000BB">feof</span><span style="color: #007700">(</span><span style="color: #0000BB">$fp</span><span style="color: #007700">)) {<br /> </span><span style="color: #0000BB">$line </span><span style="color: #007700">= </span><span style="color: #0000BB">fgets</span><span style="color: #007700">(</span><span style="color: #0000BB">$fp</span><span style="color: #007700">);<br /> </span><span style="color: #0000BB">$line </span><span style="color: #007700">= </span><span style="color: #0000BB">preg_replace_callback</span><span style="color: #007700">(<br /> </span><span style="color: #DD0000">'|<p>\s*\w|'</span><span style="color: #007700">,<br /> function (</span><span style="color: #0000BB">$matches</span><span style="color: #007700">) {<br /> return </span><span style="color: #0000BB">strtolower</span><span style="color: #007700">(</span><span style="color: #0000BB">$matches</span><span style="color: #007700">[</span><span style="color: #0000BB">0</span><span style="color: #007700">]);<br /> },<br /> </span><span style="color: #0000BB">$line<br /> </span><span style="color: #007700">);<br /> echo </span><span style="color: #0000BB">$line</span><span style="color: #007700">;<br />}<br /></span><span style="color: #0000BB">fclose</span><span style="color: #007700">(</span><span style="color: #0000BB">$fp</span><span style="color: #007700">);<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</p>
</dd>
</dt>
<dt>
<span class="term"><em><code class="parameter">subject</code></em></span>
<dd>
<p class="para">
The string or an array with strings to search and replace.
</p>
</dd>
</dt>
<dt>
<span class="term"><em><code class="parameter">limit</code></em></span>
<dd>
<p class="para">
The maximum possible replacements for each pattern in each
<em><code class="parameter">subject</code></em> string. Defaults to
<em>-1</em> (no limit).
</p>
</dd>
</dt>
<dt>
<span class="term"><em><code class="parameter">count</code></em></span>
<dd>
<p class="para">
If specified, this variable will be filled with the number of
replacements done.
</p>
</dd>
</dt>
</dl>
</p>
</div>
<div class="refsect1 returnvalues" id="refsect1-function.preg-replace-callback-returnvalues">
<h3 class="title">Return Values</h3>
<p class="para">
<span class="function"><strong>preg_replace_callback()</strong></span> returns an array if the
<em><code class="parameter">subject</code></em> parameter is an array, or a string
otherwise. On errors the return value is <strong><code>NULL</code></strong>
</p>
<p class="para">
If matches are found, the new subject will be returned, otherwise
<em><code class="parameter">subject</code></em> will be returned unchanged.
</p>
</div>
<div class="refsect1 changelog" id="refsect1-function.preg-replace-callback-changelog">
<h3 class="title">Changelog</h3>
<p class="para">
<table class="doctable informaltable">
<thead>
<tr>
<th>Version</th>
<th>Description</th>
</tr>
</thead>
<tbody class="tbody">
<tr>
<td>5.1.0</td>
<td>
The <em><code class="parameter">count</code></em> parameter was added
</td>
</tr>
</tbody>
</table>
</p>
</div>
<div class="refsect1 examples" id="refsect1-function.preg-replace-callback-examples">
<h3 class="title">Examples</h3>
<p class="para">
<div class="example" id="example-4795">
<p><strong>Example #2 <span class="function"><strong>preg_replace_callback()</strong></span> example</strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #FF8000">// this text was used in 2002<br />// we want to get this up to date for 2003<br /></span><span style="color: #0000BB">$text </span><span style="color: #007700">= </span><span style="color: #DD0000">"April fools day is 04/01/2002\n"</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$text</span><span style="color: #007700">.= </span><span style="color: #DD0000">"Last christmas was 12/24/2001\n"</span><span style="color: #007700">;<br /></span><span style="color: #FF8000">// the callback function<br /></span><span style="color: #007700">function </span><span style="color: #0000BB">next_year</span><span style="color: #007700">(</span><span style="color: #0000BB">$matches</span><span style="color: #007700">)<br />{<br /> </span><span style="color: #FF8000">// as usual: $matches[0] is the complete match<br /> // $matches[1] the match for the first subpattern<br /> // enclosed in '(...)' and so on<br /> </span><span style="color: #007700">return </span><span style="color: #0000BB">$matches</span><span style="color: #007700">[</span><span style="color: #0000BB">1</span><span style="color: #007700">].(</span><span style="color: #0000BB">$matches</span><span style="color: #007700">[</span><span style="color: #0000BB">2</span><span style="color: #007700">]+</span><span style="color: #0000BB">1</span><span style="color: #007700">);<br />}<br />echo </span><span style="color: #0000BB">preg_replace_callback</span><span style="color: #007700">(<br /> </span><span style="color: #DD0000">"|(\d{2}/\d{2}/)(\d{4})|"</span><span style="color: #007700">,<br /> </span><span style="color: #DD0000">"next_year"</span><span style="color: #007700">,<br /> </span><span style="color: #0000BB">$text</span><span style="color: #007700">);<br /><br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
<div class="example-contents"><p>The above example will output:</p></div>
<div class="example-contents screen">
<div class="cdata"><pre>
April fools day is 04/01/2003
Last christmas was 12/24/2002
</pre></div>
</div>
</div>
</p>
<p class="para">
<div class="example" id="example-4796">
<p><strong>Example #3 <span class="function"><strong>preg_replace_callback()</strong></span> using recursive structure
to handle encapsulated BB code</strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br />$input </span><span style="color: #007700">= </span><span style="color: #DD0000">"plain [indent] deep [indent] deeper [/indent] deep [/indent] plain"</span><span style="color: #007700">;<br /><br />function </span><span style="color: #0000BB">parseTagsRecursive</span><span style="color: #007700">(</span><span style="color: #0000BB">$input</span><span style="color: #007700">)<br />{<br /><br /> </span><span style="color: #0000BB">$regex </span><span style="color: #007700">= </span><span style="color: #DD0000">'#\[indent]((?:[^[]|\[(?!/?indent])|(?R))+)\[/indent]#'</span><span style="color: #007700">;<br /><br /> if (</span><span style="color: #0000BB">is_array</span><span style="color: #007700">(</span><span style="color: #0000BB">$input</span><span style="color: #007700">)) {<br /> </span><span style="color: #0000BB">$input </span><span style="color: #007700">= </span><span style="color: #DD0000">'<div style="margin-left: 10px">'</span><span style="color: #007700">.</span><span style="color: #0000BB">$input</span><span style="color: #007700">[</span><span style="color: #0000BB">1</span><span style="color: #007700">].</span><span style="color: #DD0000">'</div>'</span><span style="color: #007700">;<br /> }<br /><br /> return </span><span style="color: #0000BB">preg_replace_callback</span><span style="color: #007700">(</span><span style="color: #0000BB">$regex</span><span style="color: #007700">, </span><span style="color: #DD0000">'parseTagsRecursive'</span><span style="color: #007700">, </span><span style="color: #0000BB">$input</span><span style="color: #007700">);<br />}<br /><br /></span><span style="color: #0000BB">$output </span><span style="color: #007700">= </span><span style="color: #0000BB">parseTagsRecursive</span><span style="color: #007700">(</span><span style="color: #0000BB">$input</span><span style="color: #007700">);<br /><br />echo </span><span style="color: #0000BB">$output</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</p>
</div>
<div class="refsect1 seealso" id="refsect1-function.preg-replace-callback-seealso">
<h3 class="title">See Also</h3>
<p class="para">
<ul class="simplelist">
<li class="member"><a href="pcre.pattern.html" class="link">PCRE Patterns</a></li>
<li class="member"> <span class="function"><a href="function.preg-replace.html" class="function" rel="rdfs-seeAlso">preg_replace()</a> - Perform a regular expression search and replace</span></li>
<li class="member"> <span class="function"><a href="function.preg-last-error.html" class="function" rel="rdfs-seeAlso">preg_last_error()</a> - Returns the error code of the last PCRE regex execution</span></li>
<li class="member"><a href="functions.anonymous.html" class="link">Anonymous functions</a></li>
<li class="member">information about the <a href="language.pseudo-types.html#language.types.callback" class="link">callback</a> type</li>
</ul>
</p>
</div>
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