I try to avoid getting too tricky with regexes, because I am a bear of little brain. I'd break the string up then inspect it in pieces, perhaps like this:

my @arr = split /<\/p>/, $string;
@matches = grep { /\[tab\]/ } @arr;
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one extra line but much easier for me. If I was concerned about it looking neat, I'd put the code inside a subroutine called match_tabs().