$str = '
blah blah blah blah
blah
blah blah blah [blah
[blah blah] [blah blah blah
blah] blah]
blah blah
blah blah blah blah
blah
blah blah blah [blah
[blah blah] [blah blah blah
blah] blah]
blah blah
blah blah blah blah
blah
blah blah blah [blah
[blah blah] [blah blah blah
blah] blah]
blah blah
blah blah blah blah
blah
blah blah blah [blah
[blah blah] [blah blah blah
blah] blah]
blah blah';