Lex (software)

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.