I like this. I didn’t know about context-sensitive in regex.

It seemed the regex engine was Oniguruma/ruby.

$nc 52.69.125.71 2171 Hi! We want to hire some TRUE regular expression hackers to write firewall rules. So here are five interview questions for you, good luck! Note: After CVE-2015-4410, we reject everything contains newline, so you can just safely use ^ and$ here.

================= [SQL] =================
Please match string that contains "select" as a case insensitive subsequence.
(?i)s.*e.*l.*e.*c.*t
Running on test #885...Accepted :)

=============== [a^nb^n] ================
Yes, we know it is a classical example of context free grammer.
^(a\g<1>?b)$Running on test #370...Accepted :) ================= [x^p] ================= A prime is a natural number greater than 1 that has no positive divisors other than 1 and itself. ^(?!(xx+)\1+$)xx+$Running on test #304...Accepted :) ============= [Palindrome] ============== Both "QQ" and "TAT" are palindromes, but "PPAP" is not. ^((.)\g<1>\k<2+0>|.?)$
Running on test #799...Accepted :)

============== [a^nb^nc^n] ==============
Is CFG too easy for you? How about some context SENSITIVE grammer?
^(?=(a\g<1>b|)c)a*(b\g<2>c|)\$
Running on test #504...Accepted :)

Congratz! Here is your singing bonus:
hitcon{The pumping lemma is a lie, just like the cake}