Advance cancer cell,should be cut out with adjacent cells if it can be done.

I think this answers your concerns. If its possible you have to cut adjacent cells also. Only if cell is a advanced stage cancer.

Should I remove only next element or also previous element?

If you're logging it, you don't need to return. Or just return without logging, but in the end you'll need to return a boolean.

Issues are for problems in the kata, not problems in your solution.

???

I'm new to regex, but it seems that you don't allow for a reoccurrence of any letters or numbers.

For example:
"(?=.[a-z])" returns the first instance of any lowercase letter, but does not allow for duplicates.

"(?=.*[a-z])" returns the first instance of any lowercase letter, but the asterisk symbol allows for 0 to infinite repititions.

When I modified my code that passed all tests to remove the asterisk symbol so it look like yours, my test failed. Also, you should (from what I've read) place the "{6,}" after the "(?=.\d)" and add a "$" at the end to tell regex that the string is done (good habits to reduce errors when dealing with multiline comments if you ever do). Hopefully this helped!

Thats correct,function input is a string and output must be a string without "c" or "C" letter.When you removes "C" letter you must remove letters which are next to the letter if it possible(Important cells cant be removed).