Is a language $L$ regular, context-free, context-sensitive, recursively enumerable, or ..., if $L$ is closed understring concatenation, and/orstring repetition, and/or taking substring?Somehow related but not identical to https://cs.stackexchange.com/questions/28534/what-are-the-formal-grammars-of-the-following-recursively-defined-formal-languagFurthermore, the same question if $L$ further satisfies that every string of length 1 is also in $L$?
Is a language closed under string concatenation, repetition, and/or taking substring regular?
formal languages;closure properties