如何在没有解析器生成器的情况下用C或Objective-C编写解析器？

// OPTR stack: store operators
// OPND stack: store operands
// OP: predefined set of operators
OperandType EvaluateExpression(){  

   InitStack(OPET);Push(OPTR,\'#\');  
   initStack(OPND);c=getchar();  
   while(c!=\'#\'||GetTop(OPTR)!=\'#\'){  
     if(!In(c,OP)){Push((OPND,c);c=getchar();} //Push to stack if not operator
     else 
       switch(Precede(GetTop(OPTR),c){
         //Top element in stack has a lower priority  
         case \'<\':
               Push(OPTR,c); c=getch();  
               break;
         case \'=\':
               Pop(OPTR,x); c=getch();  
               break;
         //Pop top element and push back the calculated result 
         case \'>\':
               Pop(OPTR,theta);  
               Pop(OPND,b); Pop(OPND,a);  
               Push(OPND,Operate(a,theta,b));  
               break;  
         } 
     } 
     return GetTop(OPND);
   }

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

void expression(void);

void show(int ch) { 
    putchar(ch);
    putchar(\' \');
}

int token() { 
    int ch;
    while (isspace(ch=getchar()))
        ;
    return ch;
}

void factor() { 
    int ch = token();
    if (ch == \'(\') {
        expression();
        ch = token();
        if (ch != \')\') {
            fprintf(stderr, \"Syntax error. Expected close paren, found: %c\\n\", ch);
            exit(EXIT_FAILURE);
        }
    }
    else
        show(ch);
}

void term() { 
    int ch;
    factor();
    ch = token();
    if (ch == \'*\' || ch == \'/\') {
        term();
        show(ch);
    }
    else
        ungetc(ch, stdin);
}

void expression() {
    int ch;
    term();
    ch = token();
    if (ch == \'-\' || ch==\'+\') {
        expression();
        show(ch);
    }
    else 
        ungetc(ch, stdin);
}

int main(int argc, char **argv) {
    expression();
    return 0;
}

- (void)enumerateTagsInRange:(NSRange)range 
                      scheme:(NSString *)tagScheme
                     options:(NSLinguisticTaggerOptions)opts 
                  usingBlock:(void (^)(NSString *tag, NSRange tokenRange, NSRange sentenceRange, BOOL *stop))block