perl -00 -ne  print if $.%2==0  

The -00 flag sets the record separator to be blank lines.

Edit: to print the first multiline group:

awk  BEGIN {toggle=1} /^(space)$/ {if (!toggle) print ""; toggle=!toggle; next} {if (! toggle) print}  file.txt

Original: to print the second multiline group:

awk  /^(space)$/ { accum=""; next} /^-+$/ {print accum; accum=""; next} {accum=accum"
"$0}  file.txt

The following perl script will do what you want (I find that sed is not that well suited to tasks spanning multiple lines).

#!/usr/bin/perl

$first = 1;
$skip = 2;
while (<>) {
    chomp;
    $ln = $_;
    if ($ln =~ /^-{58}$/) {
        $skip = 2;
        next;
    }
    if ($skip > 0) {
        $skip--;
        if ($skip == 0) {
            if ($first) {
                $first = 0;
            } else {
                print "
";
            }
        }
        next;
    }
    if ($skip == 0) {
        print $ln . "
";
        if ($ln =~ /^$/) {
            $skip = -1;
        }
    }
}

This is based on the assumption that your (space) lines are just empty lines. If they re not, you will need to adjust the /^$/ pattern near the bottom to match what it actually is.

It is basically a simplified state machine controlled by the $skip variable. When this is positive, you re skipping that many lines (starts at 2 and is set to 2 for every --- line).

When $skip reaches zero, it stays there until you get an empty line (you re echoing these lines as you go). When you get an empty line, you set it to -1 and stop echoing the lines.

The $first variable is a bit of a hack to ensure there s no trailing blank line in your output.

Here s the output I got from your input file:

multiple
lines
text
(space)
different
multiple
lines
text
(space)
different
multiple
lines
text

which I believe is what you were after.

I would go awk over sed. Build a list until you hit /-+$/ and then output the multiple lines section that you stored up until each dashed line.

EDIT: I would go perl before that, but awk is fun, too.

gawk

awk   { print $2 }  RS="-
" FS="

" file

output

$ ./shell.sh
multiple
lines
text
different
multiple
lines
text
different
multiple
lines 
text

the equivalent in Perl.

$ = "
";
$/ = "-
";
while (<>) {
    chomp;
    ($f1,$f2) = split "

", $_ ;
    print $f2;
}