Move all files with number greater than argument

shift.rb

def new_filename(num, filename)
    if num.to_i < 9
        "_0#{num.to_i+1}#{filename}"
    elsif num.to_i < 99
        "_#{num.to_i+1}#{filename}"
    else
        "#{num.to_i+1}#{filename}"
    end
end

files = Dir['*']
shift = false
dry_run = false
ARGV.each do |arg|
    dry_run = true if arg == '-n'
    shift = $& if arg.to_s =~ /^[0-9]{1,4}$/
end
unless shift
    puts 'invalid parameter; exiting'
    exit 1
end
puts 'DRY RUN' if dry_run

LESS_HUNDRED = /^_([0-9]{2})(.*)/
HUNDRED_PLUS = /^([1-9][0-9]{2})(.*)/

queued_moves = []
temp_counter = 1
[LESS_HUNDRED, HUNDRED_PLUS].each do |regex|
    files.select { |file| regex =~ file }.each do |file|
        regex =~ file
        if $1.to_i > shift.to_i
            puts "renaming `#{file}` to `#{new_filename($1, $2)}`"
            File.rename file, "__temp_#{temp_counter}" unless dry_run
            queued_moves << ["__temp_#{temp_counter}", new_filename($1, $2)]
            temp_counter += 1
        end
    end
end
unless dry_run
    queued_moves.each do |move|
        File.rename move[0], move[1]
    end
end