The regular expression indicates that four backslashes are required for a backslash.

The backslash '\' in Java is usually used in combination with other characters. we sometimes call it an escape character.
To indicate a backslash '\' in Java code, it must be written as "\" in the form of a double slash. The first backslash represents an escape character, and the second can be seen as a common character.
Java does not allow the occurrence of odd backslashes. Unless there are other characters, the last backslashes are treated as escape characters.

To represent a backslash in a regular expression, four backlash lines must be used to represent "\\\\"

The four backslashes can be understood as the first two "generations" and the last two "generations.

The regular expression indicates that four backslashes are required for a backslash.