Get index of string scan results in ruby

Question

I want to get the index as well as the results of a scan

\"abab\".scan(/a/)

I would like to have not only

=> [\"a\", \"a         


        

Answer 1

It surprised me that there isn't any method similar to String#scan which would return array of MatchData objects, similar to String#match. So, if you like monkey-patching, you can combine this with Todd's solution (Enumerator is introduced in 1.9):

class Regexp
  def scan str
    Enumerator.new do |y|
      str.scan(self) do
        y << Regexp.last_match
      end
    end
  end
end
#=> nil
/a/.scan('abab').map{|m| m.offset(0)[0]}
#=> [0, 2]

Answer 2

Very similar to what @jim has said and works a bit better for longer strings:

def matches str, pattern
    arr = []
    while (str && (m = str.match pattern))      
        offset = m.offset(0).first 
        arr << offset + (arr[-1] ? arr[-1] + 1 : 0)
        str = str[(offset + 1)..-1]
    end
    arr
end

Answer 3

Try this:

res = []
"abab".scan(/a/) do |c|
  res << [c, $~.offset(0)[0]]
end

res.inspect # => [["a", 0], ["a", 2]]

Answer 4

There's a gotcha to look out for here, depending on the behaviour you expect.

If you search for /dad/ in "dadad" you'd only get [["dad",0]] because scan advances to the end of each match when it finds one (which is wrong to me).

I came up with this alternative:

def scan_str(str, pattern)
  res = []
  (0..str.length).each do |i|
    res << [Regexp.last_match.to_s, i] if str[i..-1] =~ /^#{pattern}/
  end
  res
end

If you wanted you could also do a similar thing with StringScanner from the standard library, it might be faster for long strings.