$re = '/(?<!<saw:columnHeading>).<saw:displayFormat>(.*?)<\/saw:displayFormat>.(?!<\/saw:columnHeading>)/s';
$str = '<saw:column columnID="c83" xsi:type="saw:regularColumn">
<saw:displayFormat>
<saw:formatSpec visibility="hidden" suppress="default" interaction="default" wrapText="true"/>
</saw:displayFormat>
<saw:columnHeading>
<saw:displayFormat>
<saw:formatSpec interaction="default"/>
</saw:displayFormat>
</saw:columnHeading>
<saw:columnFormula>
<sawx:expr xsi:type="sawx:sqlExpression">max("Supply Fact"."Count")</sawx:expr></saw:columnFormula>
</saw:column>';
preg_match_all($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