the Regex.Replace() method is used to replace any substring in the string str that matches the regular expression pattern @"\[.*?\]" with the replacement string "Universe". This regular expression pattern matches any substring that starts with a "[" character and ends with a "]" character, and includes any characters in between (using the ".*?" pattern). The resulting output is the string with the matched substrings replaced by the replacement string.

The Regex.Replace() method takes three arguments: the input string, the regular expression pattern, and the replacement string. The replacement string can include backreferences to capture groups in the regular expression pattern, using the syntax "$1", "$2", etc.

Note that you need to include the System.Text.RegularExpressions namespace to use the Regex class.