Lex is a computer program that generates lexical analyzers ("scanners" or "lexers").
Lex is commonly used with the yacc parser generator. Lex, originally written by Mike Lesk and Eric Schmidt and described in 1975, is the standard lexical analyzer generator on many Unix systems, and an equivalent tool is specified as part of the POSIX standard.
Lex reads an input stream specifying the lexical analyzer and writes source code which implements the lexical analyzer in the C programming language.
In addition to C, some old versions of Lex could generate a lexer in Ratfor.
Although originally distributed as proprietary software, some versions of Lex are now open-source. Open-source versions of Lex, based on the original proprietary code, are now distributed with open-source operating systems such as OpenSolaris and Plan 9 from Bell Labs. One popular open-source version of Lex, called flex, or the "fast lexical analyzer", is not derived from proprietary coding.
The structure of a Lex file is intentionally similar to that of a yacc file: files are divided into three sections, separated by lines that contain only two percent signs, as follows:
The definitions section defines macros and imports s written in C. It is also possible to write any C code here, which will be copied verbatim into the generated source file.
The rules section associates regular expression patterns with C statements. When the lexer sees text in the input matching a given pattern, it will execute the associated C code.
The C code section contains C statements and functions that are copied verbatim to the generated source file. 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.
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.
/*** Definition section **/
%{
/ C code to be copied verbatim */
#include
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We teach the fundamental aspects of analyzing and interpreting computer languages, including the techniques to build compilers. You will build a working compiler from an elegant functional language in
vignette|Éric Schmidt, programmeur de Lex.. Lex est un outil de génération d'analyseurs lexicaux en langage C. Du point de vue de la classification des langages, Lex est capable de traiter des langages de type 3 (réguliers). Il a été originellement écrit par Mike Lesk et Eric Schmidt (devenu PDG, puis président exécutif du conseil d'administration de Google) et fut décrit en 1975. Il est fréquemment utilisé en association avec Yacc, générateur d'analyseur syntaxique. Flex est la version GNU de Lex.
In computer science, the syntax of a computer language is the rules that define the combinations of symbols that are considered to be correctly structured statements or expressions in that language. This applies both to programming languages, where the document represents source code, and to markup languages, where the document represents data. The syntax of a language defines its surface form. Text-based computer languages are based on sequences of characters, while visual programming languages are based on the spatial layout and connections between symbols (which may be textual or graphical).
Flex est une version libre de l'analyseur lexical Lex. Il est généralement associé à l'analyseur syntaxique GNU Bison, la version GNU de Yacc. Flex est un outil pour générer des analyseurs, programmes qui reconnaissent des motifs lexicaux dans du texte. Il lit les fichiers d'entrée donnés, ou bien son entrée standard si aucun fichier n'est donné, pour obtenir la description de l'analyseur à générer. La description est une liste de paires d'expressions rationnelles et de code C, appelées règles.