Flex is an open source program designed to automatically and quickly generate scanners, also known as tokenizers, which recognize lexical patterns in text. Flex is an acronym that stands for "fast lexical analyzer generator. " It is a free alternative to Lex, the standard lexical analyzer generator in Unix-based systems. Flex was originally written in the C programming language by Vern Paxson in 1987.
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Lex is proprietary but versions based on the original code are available as open source. These include not only Flex but components of OpenSolaris and Plan 9. Lexical analysis is the processing of character sequences such as source code to produce symbol sequences called tokens for use as input to other programs such as parsers.