Therobs Lex & Yacc Examples and Download Links

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Overview

This page gives links to executables, reference material, and examples.

FAQ

Look at the lame, but developing, Lex and Yacc FAQ.
Questions?Any and all questions cheerfully answered by the one of therobs.

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Lex and YACC primer/HOWTO

By bert hubert

If you have been programming for any length of time in a Unix environment, you will have encountered the mystical programs Lex & YACC, or as they are known to GNU/Linux users worldwide, Flex & Bison, where Flex is a Lex implementation by Vern Paxon and Bison the GNU version of YACC. We will call these programs Lex and YACC throughout - the newer versions are upwardly compatible, so you can use Flex and Bison when trying our examples.

These programs are massively useful, but as with your C compiler, their manpage does not explain the language they understand, nor how to use them. YACC is really amazing when used in combination with Lex, however, the Bison manpage does not describe how to integrate Lex generated code with your Bison program.

There are several great books which deal with Lex & YACC. By all means read these books if you need to know more. They provide far more information than we ever will. See the 'Further Reading' section at the end. This document is aimed at bootstrapping your use of Lex & YACC, to allow you to create your first programs.

The documentation that comes with Flex and BISON is also excellent, but no tutorial. They do complement my HOWTO very well though. They too are referenced at the end.

I am by no means a YACC/Lex expert. When I started writing this document, I had exactly two days of experience. All I want to accomplish is to make those two days easier for you.

In no way expect the HOWTO to show proper YACC and Lex style. Examples have been kept very simple and there may be better ways to write them. If you know how to, please let me know.

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Lex - A Lexical Analyzer Generator

By M. E. Lesk and E. Schmidt

Lex helps write programs whose control flow is directed by instances of regular expressions in the input stream. It is well suited for editor-script type transformations and for segmenting input in preparation for a parsing routine.

Lex source is a table of regular expressions and corresponding program fragments. The table is translated to a program which reads an input stream, copying it to an output stream and partitioning the input into strings which match the given expressions. As each such string is recognized the corresponding program fragment is executed. The recognition of the expressions is performed by a deterministic finite automaton generated by Lex. The program fragments written by the user are executed in the order in which the corresponding regular expressions occur in the input stream.

The lexical analysis programs written with Lex accept ambiguous specifications and choose the longest match possible at each input point. If necessary, substantial lookahead is performed on the input, but the input stream will be backed up to the end of the current partition, so that the user has general freedom to manipulate it.

Lex can generate analyzers in either C or Ratfor, a language which can be translated automatically to portable Fortran. It is available on the PDP-11 UNIX, Honeywell GCOS, and IBM OS systems. This manual, however, will only discuss generating analyzers in C on the UNIX system, which is the only supported form of Lex under UNIX Version 7. Lex is designed to simplify interfacing with Yacc, for those with access to this compiler-compiler system.

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Debugging Lex, Yacc

cs.man.ac.uk

Typical compile-time errors
Here are the commoner error messages that you may see, with some hints as to what to do about them. Often, correcting one problem gets rid of several error messages, so fix as many as you can using these hints and your knowledge of C. If you still have error messages that aren't in this list, email me.

From Make
make: Fatal error in reader: makefile, line 8: Unexpected end of line seen

This is because you have spaces in your makefile where you should have tabs, on the previous line to the line number reported e.g. CC=gcc

test: two
two
^^^^^^^^.................. this is a tab
two: two.c
$(CC) -o two two.c
^^^^^^^^.................. but these are spaces - replace them by a tab

The first character on the lines containing commands must be a tab.

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A Compact Guide to Lex & Yacc

By Tom Niemann

epaperpress.com

This document explains how to construct a compiler using lex and yacc. Lex and yacc are tools used to generate lexical analyzers and parsers. I assume you can program in C, and understand data structures such as linked-lists and trees.

The Overview describes the basic building blocks of a compiler and explains the interaction between lex and yacc. The next two sections describe lex and yacc in more detail. With this background, we construct a sophisticated calculator. Conventional arithmetic operations and control statements, such as if-else and while, are implemented. With minor changes, we convert the calculator into a compiler for a stack-based machine. The remaining sections discuss issues that commonly arise in compiler writing.

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