Parser generators use a formal grammar to parse an input stream. To recognize more complex languages, Lex is often used with parser generators such as Yacc or Bison. Due to this, Lex can be implemented by a finite state automata as shown by the Chomsky hierarchy of languages. Lex, as with other lexical analyzers, limits rules to those which can be described by regular expressions. Using Lex with other programming tools Using Lex with parser generators This can be compiled into an executable which matches and outputs strings of integers. If this input is given to flex, it will be converted into a C file, lex.yy.c. It recognizes strings of numbers (positive integers) in the input, and simply prints them out. The following is an example Lex file for the flex version of Lex. In large programs it is more convenient to place this code in a separate file linked in at compile time. These statements presumably contain code called by the rules in the rules section.
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