[正則表達式]實用規則收錄：將一個數字按每N位加上逗號

正則：\B(?=(\d{3})+(?!\d))

以每$3$位加一個逗號爲例子，例如：

# python3
import re
pat = r'\B(?=(\d{3})+(?!\d))'
res = re.sub(pat, ',', str(12345678))
print(res) # 12,345,678

// javascript
pat = /\B(?=(\d{3})+(?!\d))/g
res = String(12345678).replace(pat, ',')
console.log(res) // 12,345,678