Validating a tab-delimited string

This code checks if string has correct format for passing to struct. Correct format needs to be [123456789'\t'First Name'\t'Last Name'\t'City]. I didn't know how to enter tabs, so I used '\t'. I use them to separate different word blocks so e.g. first and middle name can be read to a first name in struct.

I wrote alternative version as well (in comments). Which one is more preferable?
How is the correct form to write nested ternary operator

int checkString(char *string, int len)
{

    char *ch;
    int flag = FALSE;
    int i = 0;

    ch = string;

    i = 0;
    /*
    while (ch[i] && ch[i] != '\t') {
        if (isdigit(ch[i])) {
            if (++i == len) {
                flag = ch[i] == '\t' ? TRUE : FALSE;
            }
        } else { ch++; }
    }
    */

    /* ID code needs to be 11 digits */
    for (i = 0; ch[i] && ch[i] != '\t';
        isdigit(ch[i]) ? 
        flag = ++i == len ?             // while true
        ch[i] == '\t' ? TRUE : FALSE    // while true
        : FALSE 
        : *ch++);

    /* Three tabs is correct */
    if (flag) {
        for (i = 0; ch[i];
        ch[i] == '\t' ?
        flag = ++i == 3 ? TRUE : FALSE
        : *ch++);
    }

    return flag;
}

Avoid nested ternary operators. It hardly ever improves readability over proper if-else conditions.

When the boolean values true and false are involved, often there's simply no need for tensors at all, like in this example:

flag = ch[i] == '\t' ? TRUE : FALSE;

This is equivalent and simpler:

flag = ch[i] == '\t';

When a condition gets complicated, another technique is to extract the complex boolean expression to a helper method with a good descriptive name.

StackExchange Code Review Q#121837, answer score: 6