Need a regex to exclude certain strings

Question

I\'m trying to get a regex that will match:

somefile_1.txt
somefile_2.txt
somefile_{anything}.txt

but not match:

somefile_1         


        

Answer 1

The best solution has already been mentioned:

somefile_(?!16\.txt$).*\.txt

This works, and is greedy enough to take anything coming at it on the same line. If you know, however, that you want a valid file name, I'd suggest also limiting invalid characters:

somefile_(?!16)[^?%*:|"<>]*\.txt

If you're working with a regex engine that does not support lookahead, you'll have to consider how to make up that !16. You can split files into two groups, those that start with 1, and aren't followed by 6, and those that start with anything else:

somefile_(1[^6]|[^1]).*\.txt

If you want to allow somefile_16_stuff.txt but NOT somefile_16.txt, these regexes above are not enough. You'll need to set your limit differently:

somefile_(16.|1[^6]|[^1]).*\.txt

Combine this all, and you end up with two possibilities, one which blocks out the single instance (somefile_16.txt), and one which blocks out all families (somefile_16*.txt). I personally think you prefer the first one:

somefile_((16[^?%*:|"<>]|1[^6?%*:|"<>]|[^1?%*:|"<>])[^?%*:|"<>]*|1)\.txt
somefile_((1[^6?%*:|"<>]|[^1?%*:|"<>])[^?%*:|"<>]*|1)\.txt

In the version without removing special characters so it's easier to read:

somefile_((16.|1[^6]|[^1).*|1)\.txt
somefile_((1[^6]|[^1]).*|1)\.txt