$re = '/(#.+?#)([^#]+)(#.*)/m';
$str = '#4 MyApp\\Framework\\DB\\Adapter\\Pdo\\Mysql->_query(\'SELECT `store_we...\', array()) called at [vendor/myapp/framework/DB/Adapter/Pdo/Mysql.php:621]
#5 MyApp\\Framework\\DB\\Adapter\\Pdo\\Mysql->query(MyApp\\Framework\\DB\\Select#b8e969b2c2d6#, array()) called at [vendor/myapp/zendframework1/library/Zend/Db/Adapter/Abstract.php:737]
#6 Zend_Db_Adapter_Abstract->fetchAll(MyApp\\Framework\\DB\\Select#b8e969b2c2d6#) called at [vendor/myapp/module-store/App/Config/Source/RuntimeConfigSource.php:87]';
$subst = "\1StaticText\3";
$result = preg_replace($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