The main difference between Lex and Yacc is that Lex is a lexical analyzer which converts the source program into meaningful tokens while Yacc is a parser that generates a parse tree from the tokens generated by Lex.
Generally, a compiler is a software program that converts the source code into machine code. It goes through several phases to perform this conversion. First, there is a lexical analysis phase. It involves reading the source program one character at a time and converting it into meaningful lexemes (tokens). Next, the output of that phase goes to the syntax analysis. It takes the tokens of the lexical analysis and produces a parse tree as the output. Overall, the lexical analyzer performs the lexical analysis while syntax analyzer performs syntax analysis. Lex is a lexical analyzer whereas Yacc is a parser. Both work together. For example, Lex takes the string input to create tokens, and Yacc uses those tokenized input.
Key Areas Covered
Lex, Lexical Analyzer, Parse Tree, Syntax Analysis, Yacc
What is Lex
Lex is a computer program that generates lexical analyzers. Mike Lex and Eric Schmidt are the original developers of Lex. It is a standard lexical analyzer generator on various UNIX systems. Lex is specified as a part of the POSIX standard. Generally, Lex is used with Yacc parse generator. Furthermore, Lex reads an input stream specifying the lexical analyzer. Then, it outputs the source code implementing the lexer in the C language.
A Lex file consists of the following three sections:
Definition: Defines macros and imports header files written in C.
Rules: It contains regular expression patterns with C statements. When the lexer identifies that the text in the input matches a given pattern, it will execute the associated C code.
C code: This section consists of C statements and functions.
Furthermore, the open-source version of Lex is now distributed as a part of OpenSolaris and Plan 9 from Bell Labs. The most popular open-source version of Lex is called flex, which stands for Fast Lexical Analyzer.
What is Yacc
Yacc stands for Yet Another Compiler-Compiler. Stephan C. Johnson developed it, and it is used in UNIX systems. It is a standard utility on BSD and AT&T UNIX. Additionally, GNU based Linux distribution include Bison, forward-compatible Yacc replacement.
Moreover, the input to Yacc is a grammar of C code that is attached to its rules. The output is a shift-reduce parser in C. After recognizing the rule, it executes the C code associated with each rule. Further, the typical actions include generating the parse tree.
Difference Between Lex and Yacc
Lex is a computer program that operates as a lexical analyzer while Yacc is a parser that is used in the Unix Operating System.
Mike Lex and Eric Schmidt developed Lex whereas Stephan C. Johnson developed Yacc.
While Lex reads the source program one character at a time and converts it into meaningful tokens, Yacc takes the tokens as input and generates a parse tree as output.
In brief, lexical analyzer and parser support the first two phases of the compilation process. In other words, Lex is a lexical analyzer, and Yacc is a parser. Thus, the main difference between Lex and Yacc is that Lex is a lexical analyzer which converts the source program into meaningful tokens while Yacc is a parser that generates a parse tree from the tokens generated by Lex.
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