How do I find the index in a string of where my nth occurrence of a regex ends?

Question

Using Rails 5.0.1 with Ruby 2.4. How do I find the index in a string of where the nth occurrence of a regex ends? If my regex were

/\-/

and my string where

str = "a -b -c"

and I were looking for the last index of the second occurrence of my regex, I would expect the answer to be 5. I tried this

str.scan(StringHelper::MULTI_WHITE_SPACE_REGEX)[n].offset(1)

but was greeted with the error

NoMethodError: undefined method `offset' for "             ":String

In the above, n is an integer that represents the nth occurrence of the regex I wish to scan for.

Answer 1

One way of doing this:

def index_of_char str, char, n
  res = str.chars.zip(0..str.size).select { |a,b| a == char }
  res[n]&.last
end

index_of_char "a -b -c", '-', 0
#=> 2

index_of_char "a -b -c", '-', 1
#=> 5

index_of_char "a -b -c", '-', 2
#=> nil

index_of_char "abc", '-', 1
#=> nil

Further optimisations can be made.

Answer 2

Sorry about the speedy read earlier. Maybe this method can help you locate the index of the nth occorunce of an element. Although I could not find a way to do this with strictly regex in ruby. Hope this helps.

def index_of_nth_occorunce(string, element, nth_occurunce)
  count = 0
  string.split("").each_with_index do |elm, index| 
    count += 1 if elm == element
    return index if count == nth_occurunce
  end
end

index_of_nth_occorunce("a -b -c", "-", 2) #5

After doing some further digging I may have found the answer you are looking for in this stack post ( ruby regex: match and get position(s) of ). Hope this also helps.

nth_occurence = 2 
s = "a -b -c"
positions = s.enum_for(:scan, /-/).map { Regexp.last_match.begin(0) }
p positions[nth_occurence - 1] # 5

Answer 3

From my comments that grew from a link to a related question :

The answer to that question

 "abc12def34ghijklmno567pqrs".to_enum(:scan, /\\d+/).map { Regexp.last_match } 

Can easily be adapted to get the MatchData for a single item

string.to_enum(:scan, regex).map { Regexp.last_match }[n - 1].offset(0)

to find the n th match in a string.