$re = '/ns\d+:([\w-]+)/';
$str = '<?xml version="1.0"?>
<Period>
<AllContacts>
<Entry>
<ns0:entity-Person>
<ns0:CellPhone>3095550101</ns0:CellPhone>
<ns0:FirstName>Brrzzz</ns0:FirstName>
<ns0:LastName>Grbbs</ns0:LastName>
</ns0:entity-Person>
<ns0:PrimaryPhone>mobile</ns0:PrimaryPhone>
</Entry>
</AllContacts>
<State>TX</State>
</Period>';
$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