If doesn't exist or is NULL, set to and return.
如果不存在或者是NULL，就把设置为并返回。
would set variable phone to NULL.
将设置变量电话为 NULL。
This happens if you leave the event.title field null, or set it to an empty string.
如果让event . title字段为空，或者将它设置为空字符串就会出现这种情况。
The corresponding field in the business object is automatically set to null.
业务对象中的对应字段将自动设置为空。
You want to set the target buffer to NULL on failure.
一旦字符串处理函数失败，就把目标缓冲区设成NULL。
This will return null only when focus has been set to "no component."
这样，只有当焦点被设置成“无组件（no component）”时才会返回空。
If you set link to null, a new database connection is created.
如果将link设为null，就会创建一个新的数据库连接。
Every match sets a field to the captured result; every miss is set to null.
每个匹配都会把捕获结果设置到到对应的字段去；要是没有捕捉到就设为null。
Every match sets a field to the captured result; every miss is set to null.
每个匹配都会把捕获结果设置到到对应的字段去；要是没有捕捉到就设为null。
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