Analysis of information sources in references of the Wikipedia article "Flex (lexical analyser generator)" in English language version.
In about 1987, Vern Paxson of the Lawrence Berkeley Lab took a version of lex written in ratfor (an extended Fortran popular at the time) and translated it into C, calling it flex, for 'Fast Lexical Analyzer Generator.'
A freely available version of lex is flex.
This is flex, the fast lexical analyzer generator.