Yes, searchmonkey can search inside the selected file and the advanced settings and options contain ways of refining the search so that case is matched, ignored, or wildcards can even be specified. You can also construct your own regular expression to use if you have a particularly complicated search request.

You can save the search criteria and results to a file, but with searchmonkey you can't modify the file that you are searching within, and you have to open it with your text-editor as the documentation notes.

Regexxer, however, has options to search for files, and also to search and replace the search terms from a selected document or a whole series of documents. It is a more complete substitute for command-line tools like grep, sed, etc, but you could certainly use both searchmonkey and regexxer for your various purposes.

Another bonus is that regexxer integrates into Thunar very nicely, unlike searchmonkey; all you need to use in the custom actions is to add regexxer %F, and right clicking on the folder will open regexxer in that particular folder.

See my answer here for more details on regexxer and its various functions:

• Looking for a grep GUI

The screenshot shows the search within a text file with searchmonkey: