Listing with `ls` and regular expression

How about:

ls -d -- *[!0-9][0-9].txt

The ! at the beginning of the group complements its meaning.

As noted in the comments, this is bash's doing, try e.g.:

printf "%s\n" *[!0-9][0-9].txt

The question asked for regular expressions. Bash, and thus ls, does not support regular expressions here. What it supports is filename expressions (Globbing), a form of wildcards. Regular expressions are a lot more powerful than that.

If you really want to use regular expressions, you can use find -regex like this:

find . -maxdepth 1 -regex '\./.*[^0-9][0-9]\.txt'

Find is recursive by default, but ls is not. To only find files in the current directory, you can disable recursion with -maxdepth 1. Find matches against the filename with path, that's why the filenames start with ./ if you run find in .. The regex starts with \./ to cope with that. Note that in a regex, . is a special character matching any character, but here we really want a dot, that's why we escape it with a backslash. We do the same for the dot in .txt, because the regex would otherwise also match Atxt. The digit classes are same as for globbing, just that you need ^ instead of ! to invert the character class.

If you want to get the output of ls, then you can use -exec ls like this:

 find . -maxdepth 1 -regex '\./.*[^0-9][0-9]\.txt' -exec ls -lah {} \;

find supports a couple of different regex flavors. You can specify a -regextype, for example:

find . -maxdepth 1 -regextype egrep -regex '\./.*[^0-9][0-9]\.txt'

For me, possible types are: ‘findutils-default’, ‘awk’, ‘egrep’, ‘ed’, ‘emacs’, ‘gnu-awk’, ‘grep’, ‘posix-awk’, ‘posix-basic’, ‘posix-egrep’, ‘posix-extended’, ‘posix-minimal-basic’, ‘sed’ You can run find -regextype help to find out what is supported on your system.