$re = '/(id="mce_[^"]+")/';
$str = '<section id="defaultSection">
<div class="container">
<div class="row" id="mce_30" ><div class="third centerText overflowHidden"><div><img src="http://templates.weesoo.com/dummy1.jpg" class="fullWidth" data-mce-src="http://templates.weesoo.com/dummy1.jpg"></div><h3>Lorem ipsum</h3><p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p><div><a href="#" class="btnColor1 floatRight" data-mce-href="#">En savoir + </a></div></div><div class="third centerText overflowHidden"><div><img src="http://templates.weesoo.com/dummy1.jpg" class="fullWidth" data-mce-src="http://templates.weesoo.com/dummy1.jpg"></div><h3>Lorem ipsum</h3><p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p><div><a href="#" class="btnColor1 floatRight" data-mce-href="#">En savoir +</a></div></div><div class="third centerText overflowHidden"><div><img src="http://templates.weesoo.com/dummy1.jpg" class="fullWidth" data-mce-src="http://templates.weesoo.com/dummy1.jpg" data-mce-selected="1"></div><h3>Lorem ipsum</h3><p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p><div><a href="#" class="btnColor1 floatRight" data-mce-href="#">En savoir +</a></div></div><div id="mceResizeHandlenw" data-mce-bogus="all" class="mce-resizehandle" unselectable="true" style="cursor: nw-resize; margin: 0px; padding: 0px; left: 838.5px; top: -4.5px;"></div><div id="mceResizeHandlene" data-mce-bogus="all" class="mce-resizehandle" unselectable="true" style="cursor: ne-resize; margin: 0px; padding: 0px; left: 1235.296875px; top: -4.5px;"></div><div id="mceResizeHandlese" data-mce-bogus="all" class="mce-resizehandle" unselectable="true" style="cursor: se-resize; margin: 0px; padding: 0px; left: 1235.296875px; top: 293.5px;"></div><div id="mceResizeHandlesw" data-mce-bogus="all" class="mce-resizehandle" unselectable="true" style="cursor: sw-resize; margin: 0px; padding: 0px; left: 838.5px; top: 293.5px;"></div></div>
</div>
</section>
<section id="section6416597692295909"><div class="container"><div class="row editable" id="mce_31" ><div class="third centerText overflowHidden"><div><img src="http://templates.weesoo.com/dummy1.jpg" class="fullWidth" data-mce-src="http://templates.weesoo.com/dummy1.jpg"></div><h3>Lorem ipsum</h3><p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p><div><a href="#" class="btnColor1 floatRight" data-mce-href="#">En savoir + </a></div></div><div class="third centerText overflowHidden"><div><img src="http://templates.weesoo.com/dummy1.jpg" class="fullWidth" data-mce-src="http://templates.weesoo.com/dummy1.jpg"></div><h3>Lorem ipsum</h3><p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p><div><a href="#" class="btnColor1 floatRight" data-mce-href="#">En savoir +</a></div></div><div class="third centerText overflowHidden"><div><img src="http://templates.weesoo.com/dummy1.jpg" class="fullWidth" data-mce-src="http://templates.weesoo.com/dummy1.jpg"></div><h3>Lorem ipsum</h3><p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p><div><a href="#" class="btnColor1 floatRight" data-mce-href="#">En savoir +</a></div></div></div></div></section>';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
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