Tags: regex 

Rating:

Simple regex challenge.

`grep -oP flag\\{[a-z_]{16}\\} haystack.txt`

To better understand this regex, here is a [link](https://jex.im/regulex/#!flags=&re=flag%5C%5C%7B%5Ba-z_%5D%7B16%7D%5C%5C%7D)