Guess my password (robbers' thread)
Sisyphus, PHP
Password:
A string golf with 1,000,000 leading whitespaces and 1,000,000 trailing whitespaces
Output: golf
Reason: 1,000,000 is the default backtracking limit of PCRE (which you can get by var_dump(ini_get('pcre.backtrack_limit'));). And preg_match will return FALSE other than 0 or 1 when this limit is breaked.
Try it online!