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  1. /* Parenthesis Checker using Yacc/Bison and Flex */
  2.  
  3. /* parenthesis.l - Lexical analyzer (Flex input file) */
  4. %{
  5. #include <stdio.h>
  6. #include "y.tab.h"
  7. %}
  8.  
  9. %%
  10. [a-zA-Z]+   { return ID; }
  11. [(]         { return LPAREN; }
  12. [)]         { return RPAREN; }
  13. [+\-*/]     { return OP; }
  14. [ \t]       { /* ignore whitespace */ }
  15. \n          { return EOL; }
  16. .           { /* ignore other characters */ }
  17. %%
  18.  
  19. /* parenthesis.y - Parser (Yacc/Bison input file) */
  20. %{
  21. #include <stdio.h>
  22. #include <stdlib.h>
  23.  
  24. void yyerror(const char *s);
  25. int yylex();
  26. %}
  27.  
  28. %token ID OP LPAREN RPAREN EOL
  29.  
  30. %%
  31. input:
  32.     /* empty */
  33.     | input line
  34.     ;
  35.  
  36. line:
  37.     expr EOL     { printf("Balanced parentheses!\n"); }
  38.     | EOL        { /* ignore empty lines */ }
  39.     ;
  40.  
  41. expr:
  42.     ID
  43.     | LPAREN expr RPAREN
  44.     | expr OP expr
  45.     ;
  46. %%
  47.  
  48. void yyerror(const char *s) {
  49.     fprintf(stderr, "Error: syntax error\n");
  50. }
  51.  
  52. int main() {
  53.     printf("Parenthesis Checker\n");
  54.     printf("Enter expressions (Ctrl+D to exit):\n");
  55.     yyparse();
  56.     return 0;
  57. }
  58.  
  59. int yywrap() {
  60.     return 1;
  61. }
Success #stdin #stdout #stderr 0.02s 6960KB
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stdin
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Standard input is empty
stdout
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Standard output is empty
stderr
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ERROR: /home/7gBXhS/prog:5:1: Syntax error: Operator expected
ERROR: /home/7gBXhS/prog:61:0: Syntax error: Unexpected end of file
ERROR: '$runtoplevel'/0: Undefined procedure: program/0
   Exception: (3) program ? EOF: exit
https://ideone.com/XKglVH
language:
Prolog (swi 7.6.4)
created:
2 days ago
visibility:
public

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