Regex.replace [h2] to

I try to replace [h2] to <h2>
I use this coding:

string o = "<h2>test</h2>";
o = Regex.Replace(o, "[h2]", "<h2>");
o = Regex.Replace(o, "[/h2]", "</h2>");
 
But the result after the first regex is:

[<h2><h2>]test[/<h2><h2>]
So that is not what i want.
After the first 1, he should make:

<h2>test[/h2]

Whats wrong?
...