$re = '/((?<=<|<\/)|(?<= ))[A-Za-z0-9]+:| xmlns(:[A-Za-z0-9]+)?=".*?"/';
$str = '<?xml version="1.0" encoding="utf-16"?>
<ArrayOfInserts xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<insert>
<offer xmlns="http://schema.peters.com/doc_353/1/Types">0174587</offer>
<type2 xmlns="http://schema.peters.com/doc_353/1/Types">014717</type2>
<supplier xmlns="http://schema.peters.com/doc_353/1/Types">019172</supplier>
<id_frame xmlns="http://schema.peters.com/doc_353/1/Types" />
<type3 xmlns="http://schema.peters.com/doc_353/1/Types">
<type2 />
<main>false</main>
</type3>
<status xmlns="http://schema.peters.com/doc_353/1/Types">Some state</status>
</insert>
</ArrayOfInserts>';
$subst = "";
$result = preg_replace($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