I like your explanation.

However it should be noted that there are limits to "greediness". The regex will be as "greedy" as it can be while still allowing the rest of the regex to match. In this case, anchoring the regex to the "end of string" makes a difference.
/_.*_\.txt$/- the characters "gobbled up" by the .* won't include the "_.txt" at the end of the string.

Having said that, I am confused by the OPs updated comment because what we were talking about had to do the how to get the file names and not parsing the file contents itself - which is a different question!