Kleene's hierarchy

Kleene's hierarchy is a framework in computer science that classifies different types of formal languages based on their complexity. It consists of several levels: at the bottom are regular languages, which can be expressed with simple patterns. Next are context-free languages, which can describe more complex structures like nested parentheses. Above them are context-sensitive languages, capable of expressing even more intricate rules. Finally, recursively enumerable languages can include any computable language, but may not always have a clear stopping point. This hierarchy helps understand what different types of computer programs and algorithms can achieve.