$replaceAll（表达式操作符）

Operator 徽标

$replaceAll 和空值

If input or find refer to a field that is missing, they return null.

If any one of input, find, or replacement evaluates to a null, the entire $replaceAll expression evaluates to null:

$replaceAll 和排序规则

$replaceAll 表达式的字符串匹配始终区分大小写和变音符号。 使用 $replaceAll 执行字符串比较时，配置的任何排序规则都将被忽略。

例如，创建一个排序规则强度为 1 的样本集合：

db.createCollection( "restaurants", { collation: { locale: "fr", strength: 1 } } )

排序规则强度 1 仅比较基本字符并忽略其他差异，例如大小写和变音符号。

接下来，插入示例文档：

db.restaurants.insertMany( [
   { _id: 1, name: "cafe" },
   { _id: 2, name: "Cafe" },
   { _id: 3, name: "café" }
] )

以下 $replaceAll 操作尝试在 name字段中查找并替换“Cafe”的所有实例：

db.restaurants.aggregate( [
{
   $addFields:
      {
         resultObject: {
            $replaceOne: {
               input: "$name",
               find: "Cafe",
               replacement: "CAFE"
            }
         }
      }
}
] )

{ "_id" : 1, "name" : "cafe", "resultObject" : "cafe" }
{ "_id" : 2, "name" : "Cafe", "resultObject" : "CAFE" }
{ "_id" : 3, "name" : "café", "resultObject" : "café" }

Because $replaceAll ignores the collation configured for this collection, the operation only matches the instance of "Cafe" in document 2:

{ _id: 1, name: "cafe", resultObject: "cafe" }
{ _id: 2, name: "Cafe", resultObject: "CAFE" }
{ _id: 3, name: "café", resultObject: "café" }

由于该集合的排序规则强度为 1，因此遵循排序规则的操作符（例如 $match）在与“Cafe”执行字符串比较时会匹配所有三个文档。

$replaceAll 与 Unicode 规范化

The $replaceAll aggregation expression does not perform any unicode normalization. This means that string matching for all $replaceAll expressions will consider the number of code points used to represent a character in unicode when attempting a match.

例如，字符é可以使用一个或两个代码点以 unicode 表示：

Using $replaceAll with a find string where the character é is represented in unicode with one code point will not match any instance of é that uses two code points in the input string.

The following table shows whether a match occurs for a find string of "café" when compared to input strings where é is represented by either one code point or two. The find string in this example uses one code point to represent the é character:

Because $replaceAll does not perform any unicode normalization, only the first string comparison matches, where both the find and input strings use one code point to represent é.