Using Lex (Flex) to generate lexical analyzer of PL language

The first 1 Turn off ： What is? lex/flex？

I believe that after you have learned the compilation principle , Must be eager to try , Want to implement a lexical analyzer by yourself , But what? , Don't rush to do it , Learn to stand on the shoulders of giants . Did you find out , Write a large paragraph of repetition or similar scanf and if Statement to identify strings is very troublesome ？ actually , We already have lex This powerful tool .Lex yes Lexical Compiler Abbreviation , yes Unix A very famous tool in the environment , The main function is to generate a lexical analyzer (scanner) Of C Source code , Regular expressions are used to describe rules (regular expression).Flex(The Fast Lexical Analyzer) yes GNU/Linux Under the lex edition .

 /*  Simple lexical analyzer  */
 /*  function ： Be able to recognize lower case letters ab All strings at the end （ Only upper and lower case letters ） And print it 'Hit!' */
 /*  explain ： In the following begin and end Add code between , The identification of identifiers and integer constants has been realized , You need to finish the rest , Come on! ！ */
 /*  Tips ： You just need to ensure legal input （ With ab a null-terminated string ） It turns out , Illegal input will be included in . In the rules ～ */
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