Challenge Overview
How about trying to match a regular expression
The website is running here.
Solution
I started with trial and error using regex then looked at the source code at it became obvious.
The comment in the source code gives a very big hint into what it is.
// ^p.....F!?
The comment shows that it started with 'p' and ends with 'F' which based on the challenge is probably "picoCTF" (case-sensitive) which it was and it gets the flag.
"picoCTF!" and "picoCTF!?" also result in the flag.
Flag: picoCTF{succ3s...ad436ed}