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_16.txt

I tried

somefile_[^(16)].txt

with no luck (it includes even the "16" record)

Answer 1

Some regex libraries allow lookahead:

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

Otherwise, you can still use multiple character classes:

somefile([^1].|1[^6]|.|.{3,})\.txt

or, to achieve maximum portability:

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

[^(16)] means: Match any character but braces, 1, and 6.