$ STRING="(test)(te st) tesst. test test test test [teest] [teseeet]"
$ echo "$STRING" | perl -lne '    @out = ();
    /\( (.+?) \(/                 and push @out, $1;
    /.+\( \K(.+?) \. (.+?)(?= \[)/ and push @out, $1, $2;
    /(\[.+?\[)/                   and push @out, $1;
    /.+\[ (....)/                and push @out, $1;
    print join " ", @out
'
[teest] [