Is this problem in NP?
Your question makes a category error. Your problem is an enumeration problem ("List all the things with property&nbsp;X") but NP is a class of decision problems ("Does the input have property&nbsp;X?").

In any case, there is an obvious deterministic polynomial time algorithm for your problem. Just check each character of each string against&nbsp;$L$. If $L$ is fixed, this runs in linear time; if it's a part of the input, it runs in quadratic time.