Editing Yacc (section)

== Description ==
The input to Yacc is a grammar with snippets of [[C (programming language)|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 tree]]s. Using an example from Johnson, if the call {{mono|node(label, left, right)}} constructs a binary parse tree node with the specified {{mono|label}} and children, then the rule
<syntaxhighlight lang="C">
expr : expr '+' expr  { $$ = node('+', $1, $3); }
</syntaxhighlight>
recognizes summation expressions and constructs nodes for them. The special identifiers {{mono|$$}}, {{mono|$1}} and {{mono|$3}} refer to items on the parser's [[Push-down automaton|stack]].{{r|Yacc: Yet Another Compiler-Compiler}}

Yacc produces only a parser (phrase analyzer) which can be used alone in the case of [[scannerless parsing]] however, full syntactic analysis typically requires an external [[lexical analyzer]] to perform a tokenization stage first (word analysis), which is then followed by the parsing stage proper.<ref name="Yacc: Yet Another Compiler-Compiler"/> Lexical analyzer generators, such as [[Lex (software)|Lex]] or [[Flex (lexical analyser generator)|Flex]], are widely available for this purpose. The [[IEEE]] [[POSIX]] P1003.2 standard defines the functionality and requirements for both Lex and Yacc.<ref>{{man|cu|lex}}, {{man|cu|yacc}}.</ref>

Some versions of AT&T Yacc have become [[open-source software|open source]]. For example, [[source code]] is available with the standard distributions of [[Plan 9 from Bell Labs|Plan 9]].<ref>{{cite web|title=plan9: UC Berkeley release of Plan 9 under the GPLv2|website=[[GitHub]] |url=https://github.com/brho/plan9/blob/89d43d2262ad43eb4b26c2a8d6a27cfeddb33828/sys/src/cmd/yacc.c|access-date=2 January 2018|date=26 December 2017}}</ref>