OT: matched string of a regular expression

[kamaraju kusumanchi <raju.mailinglists@gmail.com>]

Consider the following file.

> cat junk.txt
a(i)1b
a(j)1b
a(ij)1b
a(ji)1b
a(ijk)1b
a(jik)1b
a(ikj)1b
a(jki)1b
a(kij)1b
a(kji)1b

In general the file is very long, might contain some other text. Now,
I would like to replace all the occurrences of

a(ijk)1b with a(ijk)23b
a(jik)1b with a(jik)23b
a(ikj)1b with a(ikj)23b
and so on for all the strings such as a(???)

Because of the size of the files involved, the number of files on
which I have to perform this operation I decided to use sed (instead
of doing it manually in vim)

However my sed script currently involves 6 lines (for all the
combinations of ijk) such as

s/a(ijk)1b/a(ijk)23b/g
s/a(jik)1b/a(jik)23b/g
s/a(ikj)1b/a(ikj)23b/g
and so on

This method is very cumbersome, not scalable. If I have to do similar
operations on a(ijklm) the script would be 120 lines! Is there any way
to write something like

s/a(???)1b/a(???)23b/g

where the second ??? is the string matched by the first regular
expression.

In general, how can I obtain the string that is matched by a regular
expression (in a shell script)?

BTW, Is sed the right tool for this kind of job? If not, can you
suggest any other tool that will get the job done in less time?

PS: Please include my email in the CC.

thanks
raju