如何在子字符串B的第一個實例之前找到子字符串A的最后一個實例？

Question

如果不需要執行正則表達式，我不想使用它。 我在文本區域中有一些文字...

<textarea>
    <!-- language-all: lang-cs -->
    [Exception filters][1] give developers the ability to add a condition (in 
    the form of a boolean expression) to a `catch` block allowing the `catch` to 
    execute only if the condition evaluates to `true`.  

    This is different from using an `if` statement inside the `catch` block and 
    re-throwing the exception since this approach stops propagating debug 
    information linked to the original exception.
</textarea>

...並且我已經選擇了我感興趣的文本行，我們稱其為substring B

var substringB = 're-throwing the exception since this approach stops propagating';

我想找到換行符（ \\n ），我們稱它為substring A ，如果存在的話，它位於substring B之前。 通過查找，我的意思是在字符串中獲取換行符的索引。

可以使用JS字符串函數完成此操作嗎？ 我們需要為此使用正則表達式嗎？

Answer 1

使用.indexOf()查找substringB ，然后使用.lastIndexOf()查找substringA從那里向后搜索。 .indexOf()和.lastIndexOf()都有一個可選的fromIndex參數。

 var text = document.querySelector("textarea").value; var substringB = 're-throwing the exception since this approach stops propagating'; var substringA = '\\n'; var subBIndex = text.indexOf(substringB); var subAIndex = text.lastIndexOf(substringA, subBIndex); console.log(subAIndex); 

 <textarea> <!-- language-all: lang-cs --> [Exception filters][1] give developers the ability to add a condition (in the form of a boolean expression) to a `catch` block allowing the `catch` to execute only if the condition evaluates to `true`. This is different from using an `if` statement inside the `catch` block and re-throwing the exception since this approach stops propagating debug information linked to the original exception. </textarea> 

注意：我顯示的簡單代碼假定將找到substringB 。 如果要在查找substringA之前進行顯式測試，可以添加if測試來檢查subBIndex != -1 。