$ltrim (expression operator)（表达式运算符）

Definition定义

$ltrim

Removes whitespace characters, including null, or the specified characters from the beginning of a string.从字符串开头删除空白字符（包括null）或指定字符。

$ltrim has the following syntax:具有以下语法：

{ $ltrim: { input: <string>,  chars: <string> } }

The $ltrim takes a document with the following fields:$ltrim接收具有以下字段的文档：

Field字段	Description描述
input	The string to trim. The argument can be any valid expression that resolves to a string. 要修剪的字符串。参数可以是解析为字符串的任何有效表达式。For more information on expressions, see Expressions.有关表达式的详细信息，请参阅表达式。
chars	Optional. 可选。The character(s) to trim from the beginning of the input.从input开头开始修剪的字符。 The argument can be any valid expression that resolves to a string. The $ltrim operator breaks down the string into individual UTF code point to trim from input.参数可以是解析为字符串的任何有效表达式。$ltrim运算符将字符串分解为单个UTF码点，以便从input中修剪。 If unspecified, $ltrim removes whitespace characters, including the null character. 如果未指定，$ltrim将删除空白字符，包括空字符。For the list of whitespace characters, see Whitespace Characters.有关空白字符的列表，请参阅空白字符。

Tip

• $trim
• $rtrim

Behavior行为

• By default, $ltrim removes whitespace characters, including the null character, from the beginning of the input string:默认情况下，$ltrim会从输入字符串的开头删除空白字符，包括空字符：

  Example示例	Results结果
  { $ltrim: { input: " \n good bye \t " } }	"good bye \t "

• You can override the default characters to trim using the chars field.您可以使用chars字段覆盖要修剪的默认字符。

  For example, the following trims any g and e from the start of the input string. Since the input starts with a whitespace, neither character can be trimmed from the start of the string.例如，以下代码从输入字符串的开头修剪任何g和e。由于输入以空格开头，因此这两个字符都不能从字符串的开头进行修剪。

  Example示例	Results结果
  { $ltrim: { input: " ggggoodbyeeeee", chars: "ge" } }	" ggggoodbyeeeee"

• If overriding the default characters to trim, you can explicitly include the whitespace character(s) to trim in the chars field.如果覆盖要修剪的默认字符，则可以在chars字段中明确包含要修剪的空白字符。

  For example, the following trims any space, g, or d from the start of the input string.例如，以下内容修剪了输入字符串开头的任何空格、g或d。

  Example示例	Results结果
  { $ltrim: { input: " ggggoodbyeeeee ", chars: " gd" } }	"oodbyeeeee "

Example示例

Consider an inventory collection with the following documents:考虑使用以下文件进行inventory集合：

db.inventory.insertMany( [
   { _id: 1, item: "ABC1", quarter: "13Q1", description: " product 1" },
   { _id: 2, item: "ABC2", quarter: "13Q4", description: "product 2 \n The product is in stock.  \n\n  " },
   { _id: 3, item: "XYZ1", quarter: "14Q2", description: null },
] )

The following operation uses the $ltrim operator to remove leading whitespaces from the description field:以下操作使用$ltrim运算符从description字段中删除前导空格：

db.inventory.aggregate([
   { $project: { item: 1, description: { $ltrim: { input: "$description" } } } }
])

The operation returns the following results:该操作返回以下结果：

{ _id: 1, item: "ABC1", description: "product 1" }
{ _id: 2, item: "ABC2", description: "product 2 \n The product is in stock.  \n\n  " }
{ _id: 3, item: "XYZ1", description: null }