Lexical analysis

In computer science, lexical analysis, lexing or tokenization is the process of converting a sequence of characters (such as in a computer program or web page) into a sequence of tokens (strings with an assigned and thus identified meaning). A program that performs lexical analysis may be termed a lexer, tokenizer, or scanner, though scanner is also a term for the first stage of a lexer. A lexer is generally combined with a parser, which together analyze the syntax of programming languages, web pages, and so forth. In computer science, lexical analysis, lexing or tokenization is the process of converting a sequence of characters (such as in a computer program or web page) into a sequence of tokens (strings with an assigned and thus identified meaning). A program that performs lexical analysis may be termed a lexer, tokenizer, or scanner, though scanner is also a term for the first stage of a lexer. A lexer is generally combined with a parser, which together analyze the syntax of programming languages, web pages, and so forth. A lexer forms the first phase of a compiler frontend in modern processing. Analysis generally occurs in one pass. In older languages such as ALGOL, the initial stage was instead line reconstruction, which performed unstropping and removed whitespace and comments (and had scannerless parsers, with no separate lexer). These steps are now done as part of the lexer. Lexers and parsers are most often used for compilers, but can be used for other computer language tools, such as prettyprinters or linters. Lexing can be divided into two stages: the scanning, which segments the input string into syntactic units called lexemes and categorizes these into token classes; and the evaluating, which converts lexemes into processed values. Lexers are generally quite simple, with most of the complexity deferred to the parser or semantic analysis phases, and can often be generated by a lexer generator, notably lex or derivatives. However, lexers can sometimes include some complexity, such as phrase structure processing to make input easier and simplify the parser, and may be written partly or fully by hand, either to support more features or for performance. A lexeme is a sequence of characters in the source program that matches the pattern for a token and is identified by the lexical analyzer as an instance of that token. Some authors term this a 'token', using 'token' interchangeably to represent the string being tokenized, and the token data structure resulting from putting this string through the tokenization process. The word lexeme in computer science is defined differently than lexeme in linguistics. A lexeme in computer science roughly corresponds to a word in linguistics (not to be confused with a word in computer architecture), although in some cases it may be more similar to a morpheme. A lexical token or simply token is a string with an assigned and thus identified meaning. It is structured as a pair consisting of a token name and an optional token value. The token name is a category of lexical unit. Common token names are