AIM:

Program to recognize a valid arithmetic expression and to recognize the identifiers and operators present. Print them separately.

DESCRIPTION:

Following the syntax for defining an identifier we generate arithmetic expressions using basic arithmetic operators such as +, -, * & /.

FLEX: flex - the fast lexical analyser generator.

CODE:

%{
#include
int a[4]={0,0,0,0};
int valid=1,i;
int nop=0;
%}
%x OPER
%%
[a-zA-Z0-9]+ {BEGIN OPER;nop++;}
"+" {if(valid) {valid=0;i=0;}else ext();}
"-" {if(valid) {valid=0;i=1;}else ext();}
"" {if(valid) {valid=0;i=2;}else ext();}
"/" {if(valid) {valid=0;i=3;}else ext();}
[a-zA-Z0-9]+ {nop++;if(valid==0){valid=1;a[i]++;}else ext();}
. {if(valid){valid=0;ext();}else return 0;}
"\n" {if(valid==0){ext();}else return 0;}
. ext();
\n ext();
%%
int ext()
{
    printf("Invalid expression\n");
    exit(0);
}
int main()
{
    printf("Enter expression\n");
    yylex();
    printf("Expression is valid\n");
    printf("No. of '+' is:%d\n",a[0]);
    printf("No. of '-' is:%d\n",a[1]);
    printf("No. of '' is:%d\n",a[2]);
    printf("No. of '/' is:%d\n",a[3]);
    printf("No. of operands is:%d\n",nop);
    return 0;
}

COMPILE:

lex 2a.l
gcc lex.yy.c -ll
./a.out 

OUTPUT:

Enter expression
2+46+2/5-36   

Expression is valid
No. of '+' is:2
No. of '-' is:1
No. of '' is:2
No. of '/' is:1
No. of operands is:7
Enter expression
2-3
Invalid expression