A formal language is a set of words in some alphabet. It may be defined as being generated by a formal grammar or as being recognized by an automaton. For a regular language, it can also be described by a regular expression.
- A regular expression is not an automaton. I wonder if it is considered as a formal grammar?
- Do other non-regular languages, such as context-free languages and recursively enumerable languages, have counterparts of regular expressions?
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