Regex capture optional group in any order - regex

Question

I would like to capture groups based on a consecutive occurrence of matched groups in any order. And when one set type is repeated without the alternative set type, the alternative set is returned as nil.
So the following:
"123 dog cat cow 456 678 890 sheep"
Would return the following:
[["123", "dog"], [nil, "cat"], ["456", "cow"], ["678", nil], ["890", sheep]]

Answer 1

A regular expression can get us part of the way, but I do not believe all the way.
r = /
(?: # begin non-capture group
\d+ # match 1+ digits
[ ] # match 1 space
[^ \d]+ # match 1+ chars other than digits and spaces
| # or
[^ \d]+ # match 1+ chars other than digits and spaces
[ ] # match 1 space
\d+ # match 1+ digits
| # or
[^ ]+ # match 1+ chars other than spaces
) # end non-capture group
/x # free-spacing regex definition mode
str = "123 dog cat cow 456 678 890 sheep"
str.scan(r).map do |s|
case s
when /\d [^ \d]/
s.split(' ')
when /[^ \d] \d/
s.split(' ').reverse
when /\d/
[s,nil]
else
[nil,s]
end
end
#=> [["123", "dog"], [nil, "cat"], ["456", "cow"],
# ["678", nil], ["890", "sheep"]]
Note:
str.scan r
#=> ["123 dog", "cat", "cow 456", "678", "890 sheep"]
This regular expression is conventionally written
/(?:\d+ [^ \d]+|[^ \d]+ \d+|[^ ]+)/
Here is another solution that only uses regular expressions incidentally.
def doit(str)
str.gsub(/[^ ]+/).with_object([]) do |s,a|
prev = a.empty? ? [0,'a'] : a.last
case s
when /\A\d+\z/ # all digits
if prev[0].nil?
a[-1][0] = s
else
a << [s,nil]
end
when /\A\D+\z/ # all non-digits
if prev[1].nil?
a[-1][1] = s
else
a << [nil,s]
end
else
raise ArgumentError
end
end
end
doit str
#=> [["123", "dog"], [nil, "cat"], ["456", "cow"], ["678", nil],
# ["890", "sheep"]]
This uses of the form of String#gsub that has no block and therefore returns an enumerator:
enum = str.gsub(/[^ ]+/)
#=> #<Enumerator: "123 dog cat cow 456 678 890 sheep":gsub(/[^ ]+/)>
enum.next
#=> "123"
enum.next
#=> "dog"
...
enum.next
#=> "sheep"
enum.next
#=> StopIteration (iteration reached an end)