You’d have to give more relation to between a file and those criteria.
Are you searching for any file that contain those ?
Will they be on the same line ?
What about the fact that file are xml ?
Are you searching for multiple tag inside a single file ?
You may be able to advance the project using a regex.
Be aware that the file in multiple file feature of atom don’t really like multiple-line regex.
A more robust solution might be to load those file in a database or database like structure and make query on that. There’s also a few data mining environment that may help you like R or some in python. Even excel to some degree.
I know you said beginning, but to some extend data mining is a hard problem and it really depend on what you are looking for, how many files, how many results per files, how many of such query will you make. How well organized or dirty the files are etc. It may be easiest to write a program with a good xml library.
It may be borderline on the responsibility of a text editor