Regex to match only innermost delimited sequence

Question

I have a string that contains sequences delimited by multiple characters: << and >>. I need a regular expression to only give me the innermost sequences. I have tried lookaheads but they don't seem to work in the way I expect them to.

Here is a test string:

'do not match this <<but match this>> not this <<BUT NOT THIS <<this too>> IT HAS CHILDREN>> <<and <also> this>>'

It should return:

but match this
this too
and <also> this

As you can see with the third result, I can't just use /<<[^>]+>>/ because the string may have one character of the delimiters, but not two in a row.

I'm fresh out of trial-and-error. Seems to me this shouldn't be this complicated.

Answer 1

@matches = $string =~ /(<<(?:(?!<<|>>).)*>>)/g;

(?:(?!PAT).)* is to patterns as [^CHAR]* is to characters.

Answer 2

$string = 'do not match this <<but match this>> not this <<BUT NOT THIS <<this too>> IT HAS CHILDREN>> <<and <also> this>>';
@matches = $string =~ /(<<(?:[^<>]+|<(?!<)|>(?!>))*>>)/g;

Answer 3

Here's a way to use split for the job:

my $str = 'do not match this <<but match this>> not this <<BUT NOT THIS <<this too>> IT HAS CHILDREN>> <<and <also> this>>';
my @a = split /(?=<<)/, $str;
@a = map { split /(?<=>>)/, $_ } @a;

my @match = grep { /^<<.*?>>$/ } @a;

Keeps the tags in there, if you want them removed, just do:

@match = map { s/^<<//; s/>>$//; $_ } @match;