Since phpBB 3.1.12 (and for boards converted/upgraded to phpBB 3.2.2 where posts weren't reparsed yet), if the board has some custom BBCodes with {URL} placeholder, there's error if you try to preview/post having any BBCode:

[phpBB Debug] PHP Warning: in file [ROOT]/includes/message_parser.php on line 84: preg_match(): Compilation failed: unrecognized character after (?< at offset 32

The error originates from 

https://github.com/phpbb/phpbb/blob/3.1.x/phpBB/includes/functions.php#L3409

https://github.com/phpbb/phpbb/blob/3.1.x/phpBB/includes/functions.php#L3419

The code was brought with the security fix https://github.com/phpbb/phpbb/commit/0aba1faa0803af6013c9ac2071e87f117e9c4835#diff-a661e9c889d836377e0341f8e90befac