番号フォーマット

main.rb

#!/usr/bin/env ruby
require 'date'

def parse(format_str)
  result = {
    date: {},
    num: false,
  }

  result[:date][:year]  = true if format_str.include?('yyyy')
  result[:date][:month] = true if format_str.include?('mm')
  result[:date][:day]   = true if format_str.include?('dd')

  if m = format_str.match(/n+/)
    result[:num] = true
  end

  result
end

def format(format_str, num: nil, date: nil)
  result = format_str

  if date
    result.gsub!('yyyy', date.strftime('%Y'))
    result.gsub!('mm',   date.strftime('%m'))
    result.gsub!('dd',   date.strftime('%d'))
  end

  if m = result.match(/n+/)
    len = m[0].size
    result.gsub!(m[0], sprintf("%0#{len}d", num))
  end

  result
end

def main
  format_str = 'yyyymmdd-nnnnn'
  num = 100
  date = Date.new(2020, 1, 1)

  pp "format_str=#{format_str}, num=#{num}, date=#{date}"
  pp parse(format_str)
  pp format(format_str, num: num, date: date)
end

main

RESULT.md

$ruby main.rb 
"format_str=yyyymmdd-nnnnn, num=100, date=2020-01-01"
{:date=>{:year=>true, :month=>true, :day=>true}, :num=>true}
"20200101-00100"