This C# article explains the concept of tokens. Programs are made out of tokens. Separating Identifier Keyword Condition Punctuators. C - Tokens and keywords; C. Rules for constructing identifier. Lexical analysis - Wikipedia, the free encyclopedia. In computer science, lexical analysis 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 identified . A program that performs lexical analysis may be called a lexer, tokenizer. Such a lexer is generally combined with a parser, which together analyze the syntax of programming languages, web pages, and so forth. A C program consists of various tokens and a token is either a keyword, an identifier. Applications. Lexing itself can be divided into two stages: the scanning, which segments the input sequence into groups and categorizes these into token classes; and the evaluating, which converts the raw input characters into a processed value. 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 partially or completely by hand, either to support additional features or for performance. A lexeme is a string of characters which forms a syntactic unit. A lexeme in computer science roughly corresponds to what in linguistics might be called a word (in computer science, 'word' has a different meaning than the meaning of 'word' in linguistics), although in some cases it may be more similar to a morpheme. A token is a structure representing a lexeme that explicitly indicates its categorization for the purpose of parsing. Examples of token categories may include . The process of forming tokens from an input stream of characters is called tokenization. Consider this expression in the C programming language: sum = 3 + 2; Tokenized and represented by the following table: Lexeme. Token categorysum. The lexical syntax is usually a regular language, with the grammar rules consisting of regular expressions; they define the set of possible character sequences that are used to form individual tokens or lexemes. A lexer recognizes strings, and for each kind of string found the lexical program takes an action, most simply producing a token. Two important common lexical categories are white space and comments. These are also defined in the grammar and processed by the lexer, but may be discarded (not producing any tokens) and considered non- significant, at most separating two tokens (as in if x instead of ifx). There are two important exceptions to this. Firstly, in off- side rule languages that delimit blocks with indentation, initial whitespace is significant, as it determines block structure, and is generally handled at the lexer level; see phrase structure, below. Secondly, in some uses of lexers, comments and whitespace must be preserved . In the 1. 96. 0s, notably for ALGOL, whitespace and comments were eliminated as part of the line reconstruction phase (the initial phase of the compiler frontend), but this separate phase has been eliminated and these are now handled by the lexer. Tokenization. The resulting tokens are then passed on to some other form of processing. The process can be considered a sub- task of parsing input.'Tokenization' has a different meaning within the field of computer security. Take, for example,The quick brown fox jumps over the lazy dog. The string isn't implicitly segmented on spaces, as an English speaker would do.
The raw input, the 4. The parser typically retrieves this information from the lexer and stores it in the abstract syntax tree. This is necessary in order to avoid information loss in the case of numbers and identifiers. Tokens are identified based on the specific rules of the lexer. Some methods used to identify tokens include: regular expressions, specific sequences of characters known as a flag, specific separating characters called delimiters, and explicit definition by a dictionary. Special characters, including punctuation characters, are commonly used by lexers to identify tokens because of their natural use in written and programming languages. Tokens are often categorized by character content or by context within the data stream. Categories are defined by the rules of the lexer. Categories often involve grammar elements of the language used in the data stream. Programming languages often categorize tokens as identifiers, operators, grouping symbols, or by data type. Written languages commonly categorize tokens as nouns, verbs, adjectives, or punctuation. Categories are used for post- processing of the tokens either by the parser or by other functions in the program. A lexical analyzer generally does nothing with combinations of tokens, a task left for a parser. For example, a typical lexical analyzer recognizes parentheses as tokens, but does nothing to ensure that each . The lexical analyzer (either generated automatically by a tool like lex, or hand- crafted) reads in a stream of characters, identifies the lexemes in the stream, and categorizes them into tokens. If the lexer finds an invalid token, it will report an error. Following tokenizing is parsing. From there, the interpreted data may be loaded into data structures for general use, interpretation, or compiling. Scanner. It has encoded within it information on the possible sequences of characters that can be contained within any of the tokens it handles (individual instances of these character sequences are known as lexemes). For instance, an integer token may contain any sequence of numerical digit characters. In many cases, the first non- whitespace character can be used to deduce the kind of token that follows and subsequent input characters are then processed one at a time until reaching a character that is not in the set of characters acceptable for that token (this is known as the maximal munch rule, or longest match rule). In some languages, the lexeme creation rules are more complicated and may involve backtracking over previously read characters. For example, in C, a single 'L' character is not enough to distinguish between an identifier that begins with 'L' and a wide- character string literal. Evaluator. In order to construct a token, the lexical analyzer needs a second stage, the evaluator, which goes over the characters of the lexeme to produce a value. The lexeme's type combined with its value is what properly constitutes a token, which can be given to a parser. Some tokens such as parentheses do not really have values, and so the evaluator function for these can return nothing: only the type is needed. Similarly, sometimes evaluators can suppress a lexeme entirely, concealing it from the parser, which is useful for whitespace and comments. The evaluators for identifiers are usually simple (literally representing the identifier), but may include some unstropping. The evaluators for integer literals may pass the string on (deferring evaluation to the semantic analysis phase), or may perform evaluation themselves, which can be involved for different bases or floating point numbers. For a simple quoted string literal, the evaluator only needs to remove the quotes, but the evaluator for an escaped string literal itself incorporates a lexer, which unescapes the escape sequences. For example, in the source code of a computer program, the stringnet. These tools generally accept regular expressions that describe the tokens allowed in the input stream. Each regular expression is associated with a production rule in the lexical grammar of the programming language that evaluates the lexemes matching the regular expression. These tools may generate source code that can be compiled and executed or construct a state table for a finite- state machine (which is plugged into template code for compilation and execution). Regular expressions compactly represent patterns that the characters in lexemes might follow. For example, for an English- based language, a NAME token might be any English alphabetical character or an underscore, followed by any number of instances of ASCII alphanumeric characters and/or underscores. This could be represented compactly by the string . It takes a full- fledged parser to recognize such patterns in their full generality. A parser can push parentheses on a stack and then try to pop them off and see if the stack is empty at the end. It is not generally considered sufficient for applications with a complicated set of lexical rules and severe performance requirements; for instance, the GNU Compiler Collection (gcc) uses hand- written lexers. Lexer generator. The most established is lex, paired with the yacc parser generator, and the free equivalents flex/bison. These generators are a form of domain- specific language, taking in a lexical specification . Further, they often provide advanced features, such as pre- and post- conditions which are hard to program by hand. However, automatically generated lexer may lack flexibility, and thus may require some manual modification or a completely manually written lexer. Lexer performance is a concern, and optimization of the lexer is worthwhile, particularly in stable languages where the lexer is run very frequently (such as C or HTML). Hand- written lexers are sometimes used, but modern lexer generators produce faster lexers than most hand- coded ones. The lex/flex family of generators uses a table- driven approach which is much less efficient than the directly coded approach. However, the lexing may be significantly more complex; most simply, lexers may omit tokens or insert additional tokens. Omitting tokens, notably whitespace and comments, is very common, when these are not needed by the compiler. Less commonly, additional tokens may be inserted. This is primarily done to group tokens into statements, or statements into blocks, to simplify the parser. Line continuation. Most frequently, ending a line with a backslash (immediately followed by a newline) results in the line being continued . What is mean by IDENTIFIER in C programming? Identifier is the fancy term used to mean . In C, identifiers are used to refer to a number of things: we've already seen them used to name variables and functions. They are also used to give names to some things we haven't seen yet, amongst which are labels and the . The rules for the construction of identifiers are simple: you may use the 5. The only restriction is the usual one; identifiers must start with an alphabetic character. Although there is no restriction on the length of identifiers in the Standard, this is a point that needs a bit of explanation. In Old C, as in Standard C, there has never been any restriction on the length of identifiers. The problem is that there was never any guarantee that more than a certain number of characters would be checked when names were compared for equality—in Old C this was eight characters, in Standard C this has changed to 3.
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