I before E except after C

# title: I before E except after C
# source: https://rosettacode.org/wiki/I_before_E_except_after_C
# category: Rosetta Code
# difficulty: Intro
# summary: Check sample words against the classic “i before e except after c” spelling rule.
# tags: popular, strings, rules, basics
# vibe: 0.2

def obeys_rule?(word)
  lower = word.downcase

  if lower.index("cie") != nil
    return false
  end

  ei_index = lower.index("ei")
  if ei_index == nil
    return true
  end

  if ei_index > 0 && lower.slice(ei_index - 1) == "c"
    return true
  end

  false
end

def run
  {
    believe: obeys_rule?("believe"),
    ceiling: obeys_rule?("ceiling"),
    science: obeys_rule?("science"),
    weird: obeys_rule?("weird"),
    ancient: obeys_rule?("ancient")
  }
end