$replaceAll (aggregation)

Operator Fields运算符字段

$replaceAll and Null Values和Null值

If input or find refer to a field that is missing, they return null.如果input或find引用了缺失的字段，则返回null。

If any one of input, find, or replacement evaluates to a null, the entire $replaceAll expression evaluates to null:如果input、find或replacement中的任何一个的计算结果为null，则整个$replaceAll表达式的计算结果将为null：

$replaceAll and Collation和排序

String matching for all $replaceAll expressions is always case-sensitive and diacritic-sensitive. Any collation configured on a collection, db.collection.aggregate(), or index is ignored when performing string comparisons with $replaceAll.所有$replaceAll表达式的字符串匹配始终区分大小写和变音符号。在与$replaceAll执行字符串比较时，将忽略在集合、db.collection.aggregate()或索引上配置的任何排序规则。

For example, create a sample collection with collation strength 1:例如，创建排序规则强度为1:

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

A collation strength of 1 compares base character only and ignores other differences such as case and diacritics.排序规则强度为1时，只比较基本字符，而忽略其他差异，如大小写和变音符号。

Next, insert three example documents:接下来，插入三个示例文档：

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

The following $replaceAll operation tries to find and replace all instances of "Cafe" in the name field:以下$replaceAll操作尝试在name字段中查找并替换“Cafe”的所有实例：

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

Because $replaceAll ignores the collation configured for this collection, the operation only matches the instance of "Cafe" in document 2:由于$replaceAll忽略为此集合配置的排序规则，因此该操作仅与文档2中的“Cafe”实例匹配：

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

Operators which respect collation, such as $match, would match all three documents when performing a string comparison against "Cafe" due to this collection's collation strength of 1.尊重排序规则的运算符（如$match）在与“Cafe”进行字符串比较时会匹配所有三个文档，因为该集合的排序规则强度为1。

$replaceAll and Unicode Normalization和Unicode规范化

The $replaceAll aggregation expression does not perform any unicode normalization. $replaceAll聚合表达式不执行任何unicode规范化。This means that string matching for all $replaceAll expressions will consider the number of code points used to represent a character in unicode when attempting a match.这意味着，在尝试匹配时，所有$replaceAll表达式的字符串匹配将考虑用于表示unicode中字符的代码点数。

For example, the character é can be represented in unicode using either one code point or two:例如，字符é可以使用一个或两个代码点在unicode中表示：

Using $replaceAll with a find string where the character é is represented in unicode with one code point will not match any instance of é that uses two code points in the input string.在查找字符串中使用$replaceAll，其中字符é在unicode中用一个代码点表示，这将与input字符串中使用两个代码点的任何é实例不匹配。

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. 下表显示了与é由一个或两个代码点表示的input字符串相比，find字符串“café”是否匹配。The find string in this example uses one code point to represent the é character:本例中的find字符串使用一个代码点来表示字符：

Because $replaceAll does not perform any unicode normalization, only the first string comparison matches, where both the find and input strings use one code point to represent é.由于$replaceAll不执行任何unicode规范化，因此只有第一个字符串比较匹配，其中find字符串和input字符串都使用一个代码点来表示é。