$re = '/(<section[a-z="\'\s]+>([\s\S]*?)<\/section>)/mi';
$str = '<chapter id="chap1">
<para0><title></title></para0>
</chapter>
<chapter id="chap2"> <title>THEORY</title>
<section id="Thoery">
<title>theory Section</title>
<para0 verstatus="ver">
<title>Theory Para 0 </title>
<text>blah blah</text>
</para0>
</section>
<section id="Next section">
<title>title</title>
<para0>
<title>Title</title>
<text>blah blah</text>
</para0>
</section>
<section id="More sections">
<title>title</title>
<para0>
<title>Title</title>
<text>blah blah</text>
</para0>
</section>
<section id="section">
<title>title</title>
<para0>
<title>Title</title>
<text>blah blah</text>
</para0>
</section>';
$subst = "$1";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php