1. use calloc instead of malloc+memset(0).
2. names and values are better to put in some "pair" struct
3. loop iterations max better to get from array size.

But I like your solution very much) This is many times better then one I write

Oh, didn't know that about the calloc, thank you for pointing this out!

result[size] = '\0'; is not needed as calloc nullifies the allocated memory.
function isupper() could be used instead of string[index] >= 'A' && string[index] <= 'Z'.

Nice solution though

FIxed in list of C katas to update

ok, looking on others people solutions I think I'm dumb

I encountered this too.

When building C code compiler returns this mistake:
fixture.c: 16: 20: warning: comparison of integers of different signs: 'int' and 'unsigned int' [-Wsign-compare]
for (int i = 0; i <input_sz; i ++)
~ ^ ~~~~~~~~
Can be easily fixed either by adding 'unsigned' into the for loop or by deleting unsinged from input_sz variable.