Lex Yacc tutorial Kun-Yuan Hsieh [email protected] Programming Tool which will produce a parser for a given grammar. Whether you use lex and yacc to build parts of your program or to build tools to aid you in programming, once you master them they will prove their worth many. Yacc (Yet Another Compiler-Compiler) is a computer program for the Unix operating system PLY (Python Lex-Yacc) is an alternative implementation of Lex and Yacc in . Compiling tools · Parser generators · Unix programming tools · Unix.


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These statements presumably contain code called by the rules in the rules section. In large programs it is more convenient to place this code in a separate file linked in at compile time.

Yacc - Wikipedia

Example of a Lex file[ edit ] The following is an example Lex file for the flex version of Lex. It recognizes strings of numbers positive integers in the input, and simply prints them out.

Or it may lex and yacc tools to reject the use of the name int as a variable name. The parser also ensures that each statement ends with a ; and that the brackets balance.


GNU has it's own, enhanced, versions called Flex and Bison. I'll keep referring to "Lex" and "Yacc", but you can use Flex and Bison as "drop-in" replacements in most cases. Yacc can optionally write a C header file containing all of the token definitions.

You include this file, called y. The Parts of Speech Lexer Example shows the declarations and rules sections of the new lexer. Lexer to be called from the parser ch There are several important differences here.

1. Lex and Yacc - lex & yacc, 2nd Edition [Book]

We have also added return statements to pass to the parser the token codes for the words that it recognizes. These return statements show that yylex acts like a coroutine. Each time the parser calls it, it takes up lex and yacc tools at the exact point it left off. This allows us to examine and operate upon the input stream incrementally.

The LEX & YACC Page

We added a rule to mark the end lex and yacc tools a sentence: The other change we made to our lexical analyzer was to omit the main routine as it will now be provided within the parser.

A Yacc Parser Finally, Example introduces our first cut at the yacc grammar. Simple yacc sentence parser ch We use it here for a C comment as with lex, C comments belong inside C code blocks, at least within the definition section and a single include file.

Lex and yacc tools come definitions of all the tokens we expect to receive from the lexical analyzer.

In this example, they correspond to the eight parts of lex and yacc tools. The name of a token does not have any intrinsic meaning to yacc, although well-chosen token names tell the reader what they represent. Although yacc lets you use any valid C identifier name for a yacc symbol, universal custom dictates that token names be all uppercase and other names in the parser mostly or entirely lowercase.

The routine yyparse is the parser generated by yacc, so our main program lex and yacc tools tries to parse sentences until the input runs out.

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