Looking for a regular expression to work on a list of comma separated values

You can simply replace the middle number of each line with nothing.

In the editor

That is, in your editor, search-and-replace the regex

,[0-9]+,

(which only matches numbers with commas on both sides, which for your input is just the middle number) with a single comma:

I assume Ultraedit supports regex search-and-replace. If not, try Notepad++, which I know does.

Since you tagged your question shell-script, here's how to do it from the command-line.

Use sed, a standard Linux command also available for Windows as part of Cygwin or GnuWin32:

C:\>sed -e 's/,[0-9]+,/,/g' filename.txt

Jens pointed out that you can also do it in Windows Powershell; see this explanation.

Regex syntax varies from application to application. I am unfamiliar with Ultredit and will give a generaql anser

Your regex lacks capturing parentheses

([0-9]+),[0-9]+,([0-9]+)

In order to replace text, one needs to tag the proper sub-expression using parentheses.

In UltraEdit, you must search for the following regular expression:

 %^([0-9]+^),[0-9]+,^([0-9]+^)