Yet another rookie question replace-regexp

[billy]

I know how to use this on a whole file. but how can I make this work on
a region?
If I want to comment out a 50 lines of 10k I don't want to comment out
each line one at a time.  I would like to do C-space bar them move
below the last line I want to effect, the do the replace-regexp.  But
his does the whole file.  I pass it  ^ / #