$re = '/^(.*?)\bdefault\b/m';
$str = '../app/code/community/Company/Module/Block/Adminhtml/Location/Grid.php
../app/code/community/Company/Module/Block/Adminhtml/Location/Import.php
../app/code/community/Company/Module/Block/Adminhtml/Location/Edit/Form.php
../app/code/community/Company/Module/Block/Adminhtml/Location/Edit/Import.php
../app/code/community/Company/Module/Block/Adminhtml/Location/Edit/Tabs.php
../app/code/community/Company/Module/Block/Adminhtml/Location/Edit/Tab/Form.php
../app/code/community/Company/Module/Block/Adminhtml/Location/Edit/Tab/Import.php
../app/code/community/Company/Module/controllers/IndexController.php
../app/code/community/Company/Module/controllers/LocationController.php
../app/design/adminhtml/default/default/layout/pmodule.xml
../app/design/frontend/default/default/layout/pmodule.xml
../app/design/frontend/default/default/template/company/module/gmap.phtml';
$subst = "\1rwd";
$result = preg_replace($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