String:

Ventilator Set:\n FiO2 ≤ 40%\nDrug use:\n NO\nWeaning trials:\n NO\nStop reason:\n neurologic factors\n備註：\n

Purpose

Find the second \n and replace it with another character

Solution

https://regex101.com/r/t9gozM/1

Pattern Explanation: 在\n後面找一個最近的\n或是結尾之間的字串

Notes:

• (?:)
  括號裡面的內容不產生指標($1之類的)
• ^
  一行的開始
• $
  一行的結束
• (?=p)
  正前向聲明，要求接下來的字元都與樣式p相符，但是不包括比對中的那些字元
• (?!p)
  反前向聲明，要求接下來的字元不與樣式p相符
  
  Route::get('/{category}/{cat_lv3}', 'ProductC@productsByCategory') ->where('cat_lv3', '^(?!\d+\-).+'); /* /products/pumps/floor-pumps match /products/floor-pumps/1322-JOEBLOW®-BOOSTER Not match */

  1 2 3 4 5 6 7 8 9 10

  Route::get('/{category}/{cat_lv3}', 'ProductC@productsByCategory')                 ->where('cat_lv3', '^(?!\d+\-).+');   /* /products/pumps/floor-pumps match   /products/floor-pumps/1322-JOEBLOW®-BOOSTER Not match */

• 重複字元(如*)後加上? – 非貪婪的重複比對
  /a+/
  /a+?/ -> 只符合一個以上的字母a

Reference:

1. Completed Usages
   https://developer.mozilla.org/zh-TW/docs/Web/JavaScript/Guide/Regular_Expressions
2. Online Test
   https://regex101.com/r/FU2s1H/1