Regular expressions, as such, only "match", they don't "extract". Some scripting languages have a facility for returning the part of a regular expression which matched, but it then depends on which language you want.

Without more information about what to look for precisely, the simple answer is that the regular expression "y" will match the letter "y", and the matching (extracted) string will always be "y".

If you want the first substring between two periods, that's something like this:

sed -n 's/.*\.\([^.]*\)\..*/\1/p' file

perl -lne 'if (m/\.([^.]*)\./) { print $1 }' file

awk -F . '{ print $2 }' file