i guided chat gpt into creating a working version of my method, it took a few tries and one tweak to get working.

splice.rb

def splice(b, e)
  if !b.is_a?(String) || !e.is_a?(String) ;  raise "Arguments require String type."
  elsif b.empty? || e.empty? ;  raise "Arguments cannot be empty."
  end
  s_copy = self.dup
  if s_copy.length <= (b.length + e.length);  raise "Base string is too small.";  end
  pos = 0 ;  stack = false ;  list = []
  if b.length > e.length ;  buffer_length = b.length
  else ;  buffer_length = e.length
  end
  
  buffer = Array.new(buffer_length, "")
  empty_buffer = Array.new(buffer_length, "")
  empty_buffer2 = Array.new(buffer_length, "")
  
  tag1 = empty_buffer
  b.split('').each do |ch|
    empty_buffer << ch
    empty_buffer.delete_at(0)
  end
  
  tag2 = empty_buffer2
  e.split('').each do |ch|
    empty_buffer2 << ch
    empty_buffer2.delete_at(0)
  end
  
  loop do
    if s_copy[pos].nil? ;  break;  end

    buffer << s_copy[pos]
    buffer.delete_at(0)
    
    if stack ;  list << s_copy[pos] ;  end
    
    str = buffer.join('')
    tag = tag1.join('').reverse
    m = true;  i = 0
    
    tag.split('').each do |ch|
      if ch != str.reverse[i] && ch != "" ;  m = false;  break ;  end
      i += 1
    end
    
    if m ; stack = true;  end
    
    tag = tag2.join('').reverse
    m = true ;  i = 0
    
    tag.split('').each do |ch|
      if ch != str.reverse[i] && ch != "";  m = false;  break;  end
      i += 1
    end
    
    if m ; stack = false;  end  
    pos += 1
  end
  
  if list.empty? ;  return nil
  else ;  return list.join('')[0..("-#{e.length + 1}").to_i]
  end 
end