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  1. %{
  2. #include <stdio.h>
  3. #include <string.h>
  4.  
  5. int op_flag = 0;        // to detect two operators in a row
  6. int valid = 1;         // assume valid until proven otherwise
  7. int balance = 0;       // parenthesis balance counter
  8. %}
  9.  
  10. %%
  11.  
  12. [0-9]+                  { op_flag = 0; }         /* number */
  13. [a-zA-Z]+               { op_flag = 0; }         /* identifier */
  14.  
  15. [+\-*/]                 {
  16.                            if(op_flag == 1) valid = 0;  // two operators in a row
  17.                            op_flag = 1;
  18.                         }
  19.  
  20. "("                     { balance++; op_flag = 0; }
  21. ")"                     { balance--; if(balance < 0) valid = 0; op_flag = 0; }
  22.  
  23. [ \t\n]+                ;   /* ignore whitespaces */
  24.  
  25. .                       { valid = 0; }  /* any other symbol → invalid */
  26.  
  27. %%
  28.  
  29. int main()
  30. {
  31.     printf("Enter mathematical expression: ");
  32.     yylex();
  33.  
  34.     if(balance != 0)
  35.         valid = 0;
  36.  
  37.     if(valid)
  38.         printf("\nValid Expression\n");
  39.     else
  40.         printf("\nInvalid Expression\n");
  41.  
  42.     return 0;
  43. }
Success #stdin #stdout #stderr 0.02s 6824KB
comments (?)
stdin 
Standard input is empty
stdout 
Standard output is empty
stderr 
ERROR: /home/U3kVPp/prog:2:1: Syntax error: Operator expected
ERROR: /home/U3kVPp/prog:43:0: Syntax error: Unexpected end of file
ERROR: '$runtoplevel'/0: Undefined procedure: program/0
   Exception: (3) program ? EOF: exit
https://ideone.com/pYVHaE
language:
Prolog (swi 7.6.4)
created:
3 days ago
visibility:
public

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