The plus is unlike the question mark and the star in that it doesn't
always necessarily succeed - because the plus requires at least
one of the character being modified. As with the star, more than this
minimum is fine.
So, if we wanted to find all the phone numbers that had just spaces
separating the groups of numbers, but only the numbers that were separated
(and not just a group of 8 numbers), then we could use the regex:
[0-9]* +[0-9]*
^ Yes, there's a space in there :-)