MatchTheRegex

This task has an input form with one field. I entered arbitrary text and intercepted the request via Burp + FoxyProxy.

Send it to Repeater for further investigation.

GET /flag?input=thisistext

I substituted various combinations into the URL to understand how the regular expression filters the input:

aaaa
flag
123123
123aaa
aaa123

And then I just thought, why not try picoCTF… And all of a sudden I was right:

I also thought about searching for a regular expression in the backend and found a hint in the Source Code:

picoCTF{succ3ssfully_matchtheregex_08c310c6}