Regex Question

I have looked over this regex a bunch of times, but still can't figure out why it is doing this.

#include <Array.au3>
$sLine = "2+-13*3"
$aRegex = StringRegExp($sLine, "(?:-)(-[\d\.]+\*-?[\d\.]+)|(?:\+)(-[\d\.]+\*-?[\d\.]+)|[\d\.]+\*-?[\d\.]+", 1)
_ArrayDisplay($aRegex)

Will return two rows with the first row being blank, but if $sLine = "2-13*3" or $sLine = "2--13*3" it returns the single row. Why is that?