Regex Golf
0x10 Classic
0x11 Warmup
Note that every word on the left contains the substring foo, while the ones on the right do not. So our answer will be foo.
0x12 Anchors
Every word on the left ends with a k while words on the right ends with other characters. Hence the answer shall be k$.
0x13 It never ends
Note that this is similar to the previous one, with u instead of k. However, $ is not allowed in this level. So we should use u\b as answer instead.
0x14 Ranges
The words on the left only contains charaters in the range a-f, while the right ones do not. So our answer shall be ^[a-f]+$.
0x15 Backrefs
In every word on the left there exists two identical substrings. To match none of the rights, the length of such substrings should be set to 3. Hence our answer is (...).*\1.