Flex (lexical analyser generator)

Flex, short for Fast Lexical Analyzer, is a computer program that generates lexical analyzers (also known as scanners or lexers). These lexical analyzers are crucial components of compilers, interpreters, and other software that needs to parse input text. Flex takes a specification file containing regular expressions and corresponding actions, and produces C or C++ source code for a scanner. This generated code, when compiled and linked with supporting routines, becomes a program that can tokenize input according to the provided regular expression rules.

The primary function of a lexical analyzer generated by Flex is to break down a stream of characters into a sequence of tokens. A token represents a meaningful unit in the input language, such as keywords, identifiers, operators, literals (numbers, strings), and punctuation. For each identified token, the generated code can perform a specified action, such as returning a token code to a parser, storing the token's value, or performing some other processing.

Flex is often used in conjunction with a parser generator such as Bison (GNU Bison), Yacc, or similar tools. In this workflow, Flex generates the scanner that provides tokens to the parser, which then uses these tokens to build a parse tree and perform semantic analysis. The combination of Flex and Bison provides a powerful and efficient way to create compilers and interpreters.

Flex offers a number of features, including support for regular expressions, multiple input buffers, start conditions (allowing for context-sensitive scanning), and customizable error handling. It provides a command-line interface for generating the scanner source code, and its performance is generally considered to be quite good, contributing to the efficiency of the resulting parsing process. Its use is widespread in both open-source and commercial software development.