正則表達式用於逃避尚未逃脫的單引號

Question

我想用\'替換'，但不是\'，因爲它們已經被轉義。

Answer 1

與lookarounds很簡單：

(?<!\\)' 

見a demo on regex101.com。
你需要躲避反斜線以及爲PHP：

<?php 
$string = "I want to replace ' with \' but not \' since they are already escaped"; 
$regex = "~(?<!\\\)'~"; 

echo preg_replace($regex, "\\'", $string); 
# Output: I want to replace \' with \' but not \' since they are already escaped 

?> 

見a demo on ideone.com。