[C]判斷一個字符串是不是合格的表達式

#include <stdio.h>
#include <ctype.h>
#define FALSE 0
#define TRUE 1
char getSymbol(char *string,int length,int *pos);
int isExpr(char * string,int length,int *pos);
int isTerm(char *string,int length,int * pos);
int isFactor(char *string,int length,int * pos);

char getSymbol(char *string,int length,int *pos){
    char c;
    if(*pos < length){
        c = string[*pos];
    }
    else{
        //printf("Overflow");
        return ' ';
    }
    (*pos)++;
    return c;
}

int isExpr(char * string,int length,int *pos){
    char symbol ;
    if(isTerm(string,length,pos) == FALSE){
        return FALSE;
    }
    if(*pos == length){
        if(isalpha(string[(*pos)-1])  || string[(*pos)-1] == ')'){
            return TRUE;
        }
        else{
            return FALSE;
        }
    }
    symbol = getSymbol(string,length,pos);
    if( symbol != '+' && symbol != '-'){
        (*pos) --;
        return TRUE;
    }
    return(isExpr(string,length,pos));
}

int isTerm(char *string,int length,int * pos){
    char symbol;
    if(isFactor(string,length,pos) == FALSE){
        return FALSE;
    }

    if(*pos == length){
        if(isalpha(string[(*pos)-1])  || string[(*pos)-1] == ')'){
            return TRUE;
        }
        else{
            return FALSE;
        }
    }
    symbol = getSymbol(string,length,pos);
    if( symbol != '*' && symbol != '/'){
        (*pos) --;
        return TRUE;
    }
    return(isTerm(string,length,pos));
}

int isFactor(char *string,int length,int * pos){
    int c;
    if(*pos == length){
        if(isalpha(string[(*pos)-1])  || string[(*pos)-1] == ')'){
            return TRUE;
        }
        else{
            return FALSE;
        }
    }
    c = getSymbol(string,length,pos);
    if(c != '('){
        return isalpha(c);
    }
    return(isExpr(string,length,pos) && getSymbol(string,length,pos) == ')');
}
int strLen(char * string){
    int len=0;
    while(string[len])
        len++;
    return len;
}
int main(void){
    char *string = "A/aaaaaa";
    int pos = 0;
    int length = strLen(string);
    if(isExpr(string,length,&pos) && pos >= length){
        printf("Valid");
    }
    else{
        printf("Invalid");
    }
    getchar();
}