Reverse a string, but preserve position of any "special" character

Gemfile

source "https://rubygems.org"

gem "minitest"

reverse.rb

def reverse input, special
  a = input.split ""
  i, j = 0, input.length - 1
  while i < j do
    if a[i] == special
      i += 1
      next
    end

    if a[j] == special
      j -= 1
      next
    end

    tmp = a[i]
    a[i] = a[j]
    a[j] = tmp
    i += 1
    j -= 1
  end
  a.join("")
end

reverse_test.rb

require 'minitest/autorun'
require "./reverse.rb"

class ReverseTest < Minitest::Test
  def test_reverse_hello
    reverse = reverse("Hello", "!")
    assert_equal reverse, "olleH"
  end

  def test_reverse_again
    reverse = reverse("Stuff and nonsense", "!")
    assert_equal reverse, "esnesnon dna ffutS"
  end

  def test_reverse_special
    reverse = reverse("Hello!", "!")
    assert_equal reverse, "olleH!"
  end

  def test_reverse_stars
    reverse = reverse "He**o", "*"
    assert_equal reverse, "oe**H"
  end
end