Yacc (Yet Another Compiler-Compiler) is a computer program for the Unix operating system developed by Stephen C. Johnson. It is a Look Ahead Left-to-Right Rightmost Derivation (LALR) parser generator, generating a LALR parser (the part of a compiler that tries to make syntactic sense of the source code) based on a formal grammar, written in a notation similar to Backus–Naur Form (BNF). Yacc is supplied as a standard utility on BSD and AT&T Unix. GNU-based Linux distributions include Bison, a forward-compatible Yacc replacement. History In the early 1970s, Stephen C. Johnson, a computer scientist at Bell Labs / AT&T, developed Yacc because he wanted to insert an exclusive or operator into a B language compiler (developed using McIlroy's TMG compiler-compiler), but it turned out to be a hard task. As a result, he was directed by his colleague at Bell Labs Al Aho to Donald Knuth's work on LR parsing, which served as the basis for Yacc. Yacc was influenced by and received its name in reference to TMG compiler-compiler. Yacc was originally written in the B programming language, but was soon rewritten in C by Alan Snyder. It appeared as part of Version 3 Unix, and a full description of Yacc was published in 1975. Johnson used Yacc to create the Portable C Compiler. Bjarne Stroustrup also attempted to use Yacc to create a formal specification of C++, but "was defeated by C's syntax". While finding it unsuitable for a formal specification of the language, Stroustrup did proceed to use Yacc to implement Cfront, the first implementation of C++. In a 2008 interview, Johnson reflected that "the contribution Yacc made to the spread of Unix and C is what I'm proudest of". Description The input to Yacc is a grammar with snippets of C code (called "actions") attached to its rules. Its output is a shift-reduce parser in C that executes the C snippets associated with each rule as soon as the rule is recognized. Typical actions involve the construction of parse trees. Using an example from Johnson, if the call constructs a binary parse tree node with the specified and children, then the rule expr : expr '+' expr { $$ = node('+', $1, $3); } recognizes summation expressions and constructs nodes for them. The special identifiers , and refer to items on the parser's stack. Yacc produces only a parser (phrase analyzer); for full syntactic analysis this