How to print lines between two patterns, inclusive or exclusive (in sed, AWK or Perl)?

Question

I have a file like the following and I would like to print the lines between two given patterns PAT1 and PAT2.

Answer 1

Using grep with PCRE (where available) to print markers and lines between markers:

$ grep -Pzo "(?s)(PAT1(.*?)(PAT2|\Z))" file
PAT1
3    - first block
4
PAT2
PAT1
7    - second block
PAT2
PAT1
10    - third block

• -P perl-regexp, PCRE. Not in all grep variants
• -z Treat the input as a set of lines, each terminated by a zero byte instead of a newline
• -o print only matching
• (?s) DotAll, ie. dot finds newlines as well
• (.*?) nongreedy find
• \Z Match only at end of string, or before newline at the end

Print lines between markers excluding end marker:

$ grep -Pzo "(?s)(PAT1(.*?)(?=(\nPAT2|\Z)))" file
PAT1
3    - first block
4
PAT1
7    - second block
PAT1
10    - third block

• (.*?)(?=(\nPAT2|\Z)) nongreedy find with lookahead for \nPAT2 and \Z

Print lines between markers excluding markers:

$ grep -Pzo "(?s)((?<=PAT1\n)(.*?)(?=(\nPAT2|\Z)))" file
3    - first block
4
7    - second block
10    - third block

• (?<=PAT1\n) positive lookbehind for PAT1\n

Print lines between markers excluding start marker:

$ grep -Pzo "(?s)((?<=PAT1\n)(.*?)(PAT2|\Z))" file
3    - first block
4
PAT2
7    - second block
PAT2
10    - third block

Answer 2

Print lines between PAT1 and PAT2

$ awk '/PAT1/,/PAT2/' file
PAT1
3    - first block
4
PAT2
PAT1
7    - second block
PAT2
PAT1
10    - third block

Or, using variables:

awk '/PAT1/{flag=1} flag; /PAT2/{flag=0}' file

How does this work?

• /PAT1/ matches lines having this text, as well as /PAT2/ does.
• /PAT1/{flag=1} sets the flag when the text PAT1 is found in a line.
• /PAT2/{flag=0} unsets the flag when the text PAT2 is found in a line.
• flag is a pattern with the default action, which is to print $0: if flag is equal 1 the line is printed. This way, it will print all those lines occurring from the time PAT1 occurs and up to the next PAT2 is seen. This will also print the lines from the last match of PAT1 up to the end of the file.

Print lines between PAT1 and PAT2 - not including PAT1 and PAT2

$ awk '/PAT1/{flag=1; next} /PAT2/{flag=0} flag' file
3    - first block
4
7    - second block
10    - third block

This uses next to skip the line that contains PAT1 in order to avoid this being printed.

This call to next can be dropped by reshuffling the blocks: awk '/PAT2/{flag=0} flag; /PAT1/{flag=1}' file.

Print lines between PAT1 and PAT2 - including PAT1

$ awk '/PAT1/{flag=1} /PAT2/{flag=0} flag' file
PAT1
3    - first block
4
PAT1
7    - second block
PAT1
10    - third block

By placing flag at the very end, it triggers the action that was set on either PAT1 or PAT2: to print on PAT1, not to print on PAT2.

Print lines between PAT1 and PAT2 - including PAT2

$ awk 'flag; /PAT1/{flag=1} /PAT2/{flag=0}' file
3    - first block
4
PAT2
7    - second block
PAT2
10    - third block

By placing flag at the very beginning, it triggers the action that was set previously and hence print the closing pattern but not the starting one.

Print lines between PAT1 and PAT2 - excluding lines from the last PAT1 to the end of file if no other PAT2 occurs

This is based on a solution by Ed Morton.

awk 'flag{
        if (/PAT2/)
           {printf "%s", buf; flag=0; buf=""}
        else
            buf = buf $0 ORS
     }
     /PAT1/ {flag=1}' file

As a one-liner:

$ awk 'flag{ if (/PAT2/){printf "%s", buf; flag=0; buf=""} else buf = buf $0 ORS}; /PAT1/{flag=1}' file
3    - first block
4
7    - second block

# note the lack of third block, since no other PAT2 happens after it

This keeps all the selected lines in a buffer that gets populated from the moment PAT1 is found. Then, it keeps being filled with the following lines until PAT2 is found. In that point, it prints the stored content and empties the buffer.