Selecting lines between marker patterns where one pattern may occur twice

If TEXT2 is always surrounded by PATTERN1 and PATTERN2 you can use grep:

grep -B2 "PATTERN2" file | grep -A1 "PATTERN1" | grep -v "PATTERN1"

• grep -B2 "PATTERN2" -> grab PATTERN2 and the preceding 2 lines
• grep -A1 "PATTERN1" -> from these three lines, grab PATTERN1 and the line after
• grep -v "PATTERN1" -> get rid of the line/s containing PATTERN1 and you are left with TEXT2

$ awk '
inBlock {
    if ( /PATTERN2/ ) {
        printf "%s", block
        inBlock = 0
    } else {
        block = block $0 ORS
    }
}
/PATTERN1/ {
    inBlock = 1
    block = ""
}
' file
TEXT2

This might work for you (GNU sed):

sed '/PATTERN1/{z;x;d};/PATTERN2/!{H;d};g;s/.//p;d' file

If the current line contains PATTERN1, clear the line and delete the hold space (HS).

If the current line does not contain PATTERN2, append it to the HS and delete the line.

If the current line contains PATTERN2, replace it by the contents of the HS, remove the first character (which will be an introduced newline), print the result and delete the line.

Alternative:

sed -En '/PATTERN1/{:a;/PATTERN1/z;N;/PATTERN2/!ba;s/.(.*)\n.*/\1/p}' file

The first solution presupposes that the file will contain PATTERN1 and PATTERN2, the second does not.

If PATTERN2 can occure multiple times, this extracts only inner text:

sed '/PATTERN1/h;//!H;/PATTERN2/!d;//{x;/PATTERN1/!d}'

If PATTERN2 can occur only once, you can use such sed script:

sed -n '/PATTERN1/h;//!H;/PATTERN2/{x;p}' input_file.txt

or:

sed '/PATTERN1/h;//!H;/PATTERN2/!d;//x'

You can reverse the lines, then use sed with 2 addresses and reverse lines again:

tac input_file.txt | sed -n '/PATTERN2/,/PATTERN1/p' | tac

With sed -z we could remove everything in front and after the patterns, since regex is greedy:

sed -z 's/.*\(PATTERN1\n\)/\1/;s/\(PATTERN2\n\).*/\1/g'

Perl to the rescue!

perl -ne 'print(@buffer), $inside = @buffer = () if /PATTERN2/;
          push @buffer, $_ if $inside;
          @buffer = (), $inside = 1 if /PATTERN1/;
' -- file.txt

We keep an array of lines to output in @buffer. We also keep a flag $inside that's set to true if we've met PATTERN1, but not PATTERN2 yet.

• If we see PATTERN2, we print the buffer and clear the flag.
• If we are inside, we remember the current line.
• If we see PATTERN1, regardless of whether we've seen it before or not, we clear the buffer and set the flag.