Nginx – Capture part of the URI and use it to do conditional rewrites in Nginx

I am writing a location block for a Nginx config, and it is for URIs with pattern like this: www.mysite.com/a/b/blah/c or www.mysite.com/a/b/blah/c/n.

Where a and b are fixed, blah may vary, c can be one of three fixed strings (optionA, optionB, optionC) only, and n is a digit starting from 0, 1, 2, and so on. In addition, n is valid only when c is equal to optionB, so that in any other case Nginx should throw a 404 immediately if there is anything after c.

I intend to write a location block as:

location ^~ /a/b/ {
    ...
}

The logic I want to implement inside this location: first capture c from a given URI, and based on its value, do one of three possible rewrites. i.e.

if (c = optionA) {
    rewrite ... ... break;
}

if (c = optionB) {
    rewrite ... ... break;
}

if (c = optionC) {
    rewrite ... ... break;
}

My questions are:

1. I have read that if statement should be generally avoided in location block in Nginx, but it seems like its use with rewrite is an exception. Will the way I use rewrite here cause any problem?

2. How do I capture c and implement the conditional rewrite logic here?