Re: regex exercise

[choroba]

/m isn't needed, as you don't use ^ or $ in the regex. /i isn't needed, either, as <p> is always lowercase in the sample data.

I also used s%%% instead of s/// to avoid the need to backslash the slashes in endtags.

s%(<p>(?=(?:(?!</p>).)*\n\n)(?:(?!</p>).)*</p>)%-->$1<--%sg

Explanation:

The p-tag must be followed by two newlines that aren't preceded by its endtag.

You can make the regex more readable via /x :

    s{ (<p>
         (?=(?:(?!</p>).)*\n\n) # Followed by two newlines not precede
+d by </p>
         (?:(?!</p>).)*</p>)    # Followed by </p> not preceded by </p
+>
     }{-->$1<--}sgx;

Update: or even

my $not_followed_by_end_p = qr{(?:(?!</p>).)*}s;
while (<>) {
    s{ (<p>
         (?=$not_followed_by_end_p\n\n)
         $not_followed_by_end_p</p>)
     }{-->$1<--}gx;
    print;
}