perl -0pe 's/pattern1|pattern2/$1/gs'; apparently the "or" operator doesn't work as expected here.

The | alternation operator has pretty low precedence, so it kind of depends on what your expectations are :-) A common trap is to write something like /^foo|bar$/ and expect that to match only "foo" or "bar", when in fact it is matching ^foo or bar$ - the correct way to express that would have been /^(foo|bar)$/ or /^(?:foo|bar)$/.

Based on your $1 in the replacement, I suspect you were doing something like s/f(o)o|b(a)r/$1/g and expecting the string "bar" to be turned into "a"? In that case, you need the "branch reset" pattern (?|...) (perlre): s/(?|f(o)o|b(a)r)/$1/g will replace "foo" with "o" and "bar" with "a".

Thank you, haukex, for the information about the correct use of the branch reset, and sorry for making you guess my incorrect code. It was (and is) a bit of a moot point, since I've been given multiple perl solutions to my problem; however, for the record, my aforementioned failed attempt was to combine the ideas I had for what in my original post I described as the former and latter cases:

perl -0pe 's/(.*?\n*?([^ \n].*?)\n\n.*|.*?\n([^ \n].*?)\n\n.*)/$1/gs' 
+file.txt
[download]

This simply returned the whole file, and I thought it had something to do with the nested parentheses altering what $1 refers to. When I rewrite it correctly, as per haukex's helpful advice:

perl -0pe 's/(?|.*?\n*?([^ \n].*?)\n\n.*|.*?\n([^ \n].*?)\n\n.*)/$1/gs
+' file.txt
[download]

it does return something, but that something is the desired output only in the case where the first line is non-indented. The reason, obviously, is that I was making a stupid mistake: the first alternative returns a match whether or not the first line of file.txt is indented, so that code is never going to return the output of:

perl -0pe 's/.*?\n([^ \n].*?)\n\n.*/$1/gs' file.txt
[download]

As I said, it's a moot point, but I thought this follow-up was worth mentioning in case someone who lands on this page learns from my stupid mistake.

Regards,

Maneesh

s/(?:foo|bar)/baz/;
[download]

s/(?:foo|bar)/baz/ is pretty much identical to s/foo|bar/baz/, so I don't think that's the issue.