Description
Regular expressions are pretty useful. Especially when you need to search and replace complex terms.
Solution
The site appeared to be a form to perform regular expression replacement
A simple test verified my initial thoughts
From the placeholder in the pattern field, it also indicated that it’s possible
to use arbitrary modifiers as well. Having that in mind, it appears this application
might be vulnerable to exploiting the applipcation through the /e modifier
which works as eval().
This modifier is deprecated since PHP 5.5.0 - preg_replace()
This proved my theory, and it was trivial to extract the flag.
<?php $FLAG = "IW{R3Pl4c3_N0t_S4F3}"; ?> arFlag: IW{R3Pl4c3_N0t_S4F3}