C# replacing string between arrays

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C#7.0

Ok I don't feel like the question is self-explanatory so I will explain it.
I'm making a program that requires relocating the data, but it reads a line one by one so I need to store things to replace and to replace it with in a array.
Lets say I need to replace the string
"test"
with
"test2"
well my array would be set up like this:

string[] arr = new string[10];
arr[1] = "test";
arr[2] = "test2";

That would be the setup.
Here's my code for this:

string text = kek.Text;

for (int idx = 0; idx < reloc.Length; idx++)
{
    text = Regex.Replace(text, reloc[idx], reloc[idx + 1]);
}
return text;

I was wondering how would I do this correctly as the error I'm getting right now is:
"Value Cannot be Null Parameter Name Pattern"

What I have tried:

Rewriting my method.
Adjusting with simple math.