How to replace the last occurrence of a substring in ruby?

Question

I want to replace the last occurrence of a substring in Ruby. What\'s the easiest way? For example, in abc123abc123, I want to replace the last abc

Answer 1

Since Ruby 2.0 we can use \K which removes any text matched before it from the returned match. Combine with a greedy operator and you get this:

'abc123abc123'.sub(/.*\Kabc/, 'ABC')
#=> "abc123ABC123"

This is about 1.4 times faster than using capturing groups as Hirurg103 suggested, but that speed comes at the cost of lowering readability by using a lesser-known pattern.

more info on \K: https://www.regular-expressions.info/keep.html

Answer 2

I've used this handy helper method quite a bit:

def gsub_last(str, source, target)
  return str unless str.include?(source)
  top, middle, bottom = str.rpartition(source)
  "#{top}#{target}#{bottom}"
end

If you want to make it more Rails-y, extend it on the String class itself:

class String
  def gsub_last(source, target)
    return self unless self.include?(source)
    top, middle, bottom = self.rpartition(source)
    "#{top}#{target}#{bottom}"
  end
end

Then you can just call it directly on any String instance, eg "fooBAR123BAR".gsub_last("BAR", "FOO") == "fooBAR123FOO"

Answer 3

You can achieve this with String#sub and greedy regexp .* like this:

'abc123abc123'.sub(/(.*)abc/, '\1ABC')

Answer 4

string = "abc123abc123"
pattern = /abc/
replacement = "ABC"

matches = string.scan(pattern).length
index = 0
string.gsub(pattern) do |match|
  index += 1
  index == matches ? replacement : match
end
#=> abc123ABC123