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<?php echo preg_replace('`(?={)([^{}]*|{[^{}]*})+(?=})`', '<$0>', 'test {a} {b{c}d}e{{f}{g}}');
Output for 4.3.0 - 5.6.28, hhvm-3.10.0 - 3.13.2, 7.0.0 - 7.1.0
test {a} {b<{c}d>}e{<{f}{g}>}