I am having a tricky regex issue

I have the string like below

```
some_Name   _  _Bday Date Comm.txt

```

And here is my regex to match the spaces and underscore

```
\_?\s\_?

Now when i try to replace the string using sed and the above regex

echo "some_Name   _  _Bday Date Comm.txt" | sed 's/\_?\s\_?/\_/g'

```

The output i want is

```
some_Name_Bday_Date_Comm.txt

```

Any ideas on how do i go about this ?