how do you specify non-capturing groups in sed?

Parentheses can be used for grouping alternatives. For example:

sed 's/a\(bc\|de\)f/X/'

says to replace "abcf" or "adef" with "X", but the parentheses also capture. There is not a facility in sed to do such grouping without also capturing. If you have a complex regex that does both alternative grouping and capturing, you will simply have to be careful in selecting the correct capture group in your replacement.

Perhaps you could say more about what it is you're trying to accomplish (what your need for non-capturing groups is) and why you want to avoid capture groups.

Edit:

There is a type of non-capturing brackets ((?:pattern)) that are part of Perl-Compatible Regular Expressions (PCRE). They are not supported in sed (but are when using grep -P).

The answer, is that as of writing, you can't - sed does not support it.

Non-capturing groups have the syntax of (?:a) and are a PCRE syntax.

Sed supports BRE(Basic regular expressions), aka POSIX BRE, and if using GNU sed, there is the option -r that makes it support ERE(extended regular expressions) aka POSIX ERE, but still not PCRE)

Perl will work, for windows or linux

examples here

https://superuser.com/questions/416419/perl-for-matching-with-regular-expressions-in-terminal

e.g. this from cygwin in windows

$ echo -e 'abcd' | perl -0777 -pe 's/(a)(?:b)(c)(d)/\1/s'
a

$ echo -e 'abcd' | perl -0777 -pe 's/(a)(?:b)(c)(d)/\2/s'
c

There is a program albeit for Windows, which can do search and replace on the command line, and does support PCRE. It's called rxrepl. It's not sed of course, but it does search and replace with PCRE support.

C:\blah\rxrepl>echo abc | rxrepl -s "(a)(b)(c)" -r "\1"
a

C:\blah\rxrepl>echo abc | rxrepl -s "(a)(b)(c)" -r "\3"
c

C:\blah\rxrepl>echo abc | rxrepl -s "(a)(b)(?:c)" -r "\3"
Invalid match group requested.

C:\blah\rxrepl>echo abc | rxrepl -s "(a)(?:b)(c)" -r "\2"
c

C:\blah\rxrepl>

The author(not me), mentioned his program in an answer over here https://superuser.com/questions/339118/regex-replace-from-command-line

It has a really good syntax.

The standard thing to use would be perl, or almost any other programming language that people use.