Take this string.
a = "real-ab(+)real-bc(+)real-cd-xy"
a.scan(/[a-z_0-9]+\-[a-z_0-9]+[\-\[a-z_0-9]+\]?/)
=> ["real-ab", "real-bc", "real-cd-xy"]
But how come this next string gets nothing?
a = "real-a(+)real-b(+)real-c"
a.scan(/[a-z_0-9]+\-[a-z_0-9]+[\-\[a-z_0-9]+\]?/)
=> []
How can I have it so both strings output into a 3 count array?
You've confused parentheses (used for grouping) and square brackets (used for character classes). You want
a.scan(/[a-z_0-9]+-[a-z_0-9]+(?:-[a-z_0-9]+)?/)
(?:...) creates a non-capturing group which is what you need here.
Furthermore, unless you want to disallow uppercase letters explicitly, you can write \w as a shorthand for "a letter, digit or underscore":
a.scan(/\w+-\w+(?:-\w+)?/)
a.scan(/[a-z_0-9]+\-[a-z_0-9]+/)
real-cd-xy completely.Why not simply?
a.scan(/[a-z_0-9\-]+/)
--- or real- or -real, all of which are not legal by his first regex.