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