Hello,
from the description of the project it doesn't seem to be a complex software, but it has to be engineered well, because of the extensibility You desire. I can implement the basic program in Python itself using the Qt5 framework, a choice which will allow to quickly prototype the application and make it also cross-platform at no cost (aiming to make it also distributable on platforms other than OSX, like Linux and Windows). If I got it correctly, You have some knowledge of Python, this will even allow you to modify and extend the app on your own in future. I have spent some time brainstorming a bit the idea, and this is how it looks like:
1. A simple UI (list-like) which allows the user to create expressions and give them a name.
2. The UI allows also to change the order of the expressions by drag-n-drop.
3. A facility for file upload, to start simple, TXT files, other formats can be added later with 3rd-party libraries (I have experience with word/pdf documents reading).
4. A "Run" button, which executes in order all expressions in the list and under each one lists the matched results.
5. An "Export" button, which picks up all the results for each expression, flats them one per column and exports the whole set as a CSV file. Optionally, each expression widget could have a checkbox, in order to allow only partial exports.
6. Saving and loading of the expression collections.
7. (depends on time remaining) Ability to process multiple documents at once.