Regex issue with replace simple string

Regex issue with replace simple string

I am using Regex replace as below :
string test = "[abc] Sends the employee mail. [twist] Sends the employee
mail.";
test = Regex.Replace(test, "[twist]", "hello");
And the result is coming as :
test = "[abc] Sendhello hellohe employee mahellol.
[hellohellohellohellohello] Sendhello hellohe employee mahellol."
Where as it should just replace [twist] string with hello.
What is going wrong here.