我想用''而不是''替换'，因为它们已经转义了。
使用查找非常简单：
(?<!'')'
请参阅regex101.com上的演示。
对于PHP:，您还需要转义反斜杠
<?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
?>
请参阅ideone.com.