Lexemes, lexical categories, and tokens

Programming languages read characters and group adjacent characters together when they are part of the same entity in the language. This can be a multi-character name or reserved word, a constant value, or an operator.A lexeme is a string of adjacent characters that form a single entity. Most punctuation marks are lexemes unto themselves, in addition to separating what came before from what comes after them. In reasonable languages, whitespace characters such as spaces and tabs are ignored other than to separate lexemes. Almost all languages also have a way of including comments in the source code, and comments are typically treated the same as whitespace: they can be the boundary that separates two lexemes, but they are discarded and not considered further.Each lexeme has a lexical category. In natural languages, lexical categories are called parts of speech: noun, verb, and adjective are examples. In a programming language implementation, the...