Find deepest nested paranthesis

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

char* findInnermost(char* stream);

int main()
{
    char* test = "hopefully (the thing returned (should (be (this))))(and not this)";
    char* result = findInnermost(test);
    printf("%s\n", result);
    free(result);
    return 0;
}

char* findInnermost(char* stream)
{
    int depth = 0, maxDepth = 0;
    char *leftptr = stream, *rightptr = stream + strlen(stream), *currentptr;

    for (currentptr = leftptr; *currentptr != '\0'; currentptr++)
    {
        switch (*currentptr)
        {
            case '(':
                depth++;

                if (depth >= maxDepth)
                {
                    maxDepth = depth;
                    leftptr = currentptr;
                }
                break;

            case ')':
                if (depth == maxDepth) rightptr = currentptr;
                depth--;
                break;
        }
    }

    char* innermost = malloc(sizeof(char) * ((rightptr - leftptr) - 1));
    strncpy(innermost, leftptr + 1, rightptr - leftptr - 1);
    innermost[rightptr - leftptr] = '\0';
    return innermost;
}