<?php
$input = '[Lorem ipsum]
Dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh euismod tincidunt ut laoreet dolore magna aliquam erat volutpat.
[Ut wisi]
[Enim ad minim veniam](a), quis nostrud exerci tation ullamcorper.
suscipit lobortis nisl ut aliquip ex ea commodo consequat. Duis autem vel eum iriure dolor in hendrerit in vulputate velit esse molestie consequat.
Vel illum dolore eu feugiat nulla facilisis at vero.
[Ros et accumsan et iusto odio dignissim](b) qui blandit praesent luptatum zzril delenit augue duis dolore te feugait nulla facilisi.
[[Nam liber]](c)
Tempor cum soluta nobis eleifend option congue nihil imperdiet doming id quod mazim placerat facer possim assum.';
\[( # open outer square brackets and capturing group
(?: # open subpattern for optional inner square brackets
[^[\]]* # non-square-bracket characters
\[ # open inner square bracket
[^[\]]* # non-square-bracket characters
] # close inner square bracket
)* # end subpattern and repeat it 0 or more times
[^[\]]* # non-square-bracket characters
)] # end capturing group and outer square brackets
(?: # open subpattern for optional parentheses
\(( # open parentheses and capturing group
[a-z]+ # letters
)\) # close capturing group and parentheses
)? # end subpattern and make it optional
~isx",
$input,
$matches);
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