lex yacc tutorial by Tom Niemann

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KTU Compiler Lab CSL411 - Dr Binu V P

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Preamble:  This course aims to offer students hands-on experience on compiler design concepts.Students will be able to familiarize with tools such as LEX and YACC and automate different phases of a compiler. This course helps the learners to enhance the capability to design and implement a compiler. Prerequisite: A sound knowledge in C programming, Data Structures, Formal languages and Automata Theory and Compiler design. About Me Syllabus and practise questions Lex yacc tutorial by Tom Neimann How to Install  flex and Bison Introduction to lex/flex Format of lex input files Lex Regular Expressions Input Matching in Lex Actions in Lex File Start Conditions in Lex Values available to the user in action section Reading and writing files Lex/Flex  Programs     Simple lexical analyser in C     count number of lines ,words and characters     check for valid identifiers     count the identifiers     converting the substring "abc" to "ABC"          count vowels and consona
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lexical analyzer for a c program

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 This program identifies tokens in a C program ignoring white spaces and comments.The input C program is given as argument ******************************* %{ #include <stdio.h> %} %option noyywrap /* Define regular expressions for different token types */ DIGIT      [0-9] LETTER     [a-zA-Z] IDENTIFIER {LETTER}({LETTER}|{DIGIT})* NUMBER     {DIGIT}+ WHITESPACE [ \t\n]+ COMMENT1    "//".* COMMENT2  "/*"([^*]|\*+[^/*])*\*+"/" %% {WHITESPACE}        { /* Ignore whitespace */ } {COMMENT1}           { /* Ignore comments */ } {COMMENT2}           { /* Ignore comments */ } "int"               { printf("TOKEN_INT\n"); } "return"            { printf("TOKEN_RETURN\n"); } "if"                { printf("TOKEN_IF\n"); } "else"              { printf("TOKEN_ELSE\n"); } "while"             { printf("TOKEN_WHILE\n"); } "for"               { printf("TOKEN_
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13.Precedence and conflict resolution in yacc

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We had seen that , when you feed yacc a grammar that is not LALR(1), it shows a warning and resolve the conflicts automatically. For a shift/reduce conflict, yacc will choose the shift. In a reduce/reduce conflict, it will reduce using the rule declared first in the file. We can control what happens by explicitly declaring precedence and associativity for operators. Rather than re-writing the grammar to implicitly control the precedence and associativity with a bunch of intermediate non-terminals, we can directly indicate the precedence so that yacc will know how to break ties. In the definitions section, we can add any number of precedence levels, one per line, from lowest to highest, and indicate the associativity (either left, right, or nonassoc). Several terminals can be on the same line to assign them equal precedence. Example: %token T_Int %left '+' %left '*' %right '^' %% E : E '+' E |E '*'
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