Multiple validation under single input type

I am working on a project. I have used [a-zA-Z]+ for only alphabetic input. I want to restrict a user to start with only (BS- or MSC-) and then input anything Below is the input type:

 <input type="text" class="form-control" placeholder="Enter Degree Name Example BS-IT" name="degree" required pattern="[a-zA-Z]+" title="Example BS-IT">