Bug or feature?

[Eric Cloninger]

# echo " 1 2 3 4" |grep -o "^ [0-9]"
 1
 2
 3
 4

It would appear that -o chops up the input stream into a new line every time a 
match is found.  However, I expect a "^...." grep to find only one result per 
line of input, regardless of the return method (whole line or only-matching).  
Bug or feature?