Open source and Versions
– Some versions of Lex are now open-source.
– Open-source versions of Lex are distributed with open-source operating systems like OpenSolaris and Plan 9 from Bell Labs.
– The popular open-source version of Lex is called flex.
– Flex is not derived from proprietary coding.
– Flex is known as the fast lexical analyzer.
Structure of a Lex file
– Lex files are divided into three sections: definitions, rules, and C code.
– The definitions section defines macros and imports header files written in C.
– C code can be written in the definitions section and will be copied into the generated source file.
– The rules section associates regular expression patterns with C statements.
– When the lexer sees text matching a pattern, it executes the associated C code.
Example of a Lex file
– An example Lex file for the flex version of Lex recognizes strings of numbers and prints them out.
– The file has a definitions section, a rules section, and a C code section.
– The rules section uses regular expressions to match strings of digits.
– The C code section contains the main function that calls the lexer.
– The input given to flex will be converted into a C file called lex.yy.c.
Using Lex with other programming tools
– Lex can be used with parser generators like Yacc or Bison.
– Lex limits rules to those describable by regular expressions.
– Parser generators use a formal grammar to parse an input stream.
– Lex is often used to produce a token-stream for parser generators.
– Scannerless parsing refers to parsing the input character-stream directly, without a distinct lexer.
Lex and make
– Make is a utility used to maintain programs involving Lex.
– Make assumes that a file with the extension ‘.l’ is a Lex source file.
– The make internal macro LFLAGS can be used to specify Lex options for make.
– Lex can be integrated into the make workflow for automatic compilation.
– Make simplifies the build process for Lex programs.
|Mike Lesk, Eric Schmidt
|Unix, Unix-like, Plan 9
|Plan 9: MIT License
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.
In addition to C, some old versions of Lex could generate a lexer in Ratfor.
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