regex - In Java Regular Expression "1" (back reference) is not working

I want to replace string "Cannot" with "Can not" and "cannot" with "can not". For that, I am using the code below:

 String string = "I Cannot do it.";
 string = string.replaceAll("([Cc])annot", "\\1an not");

Desired string is "I Can not do it.".

String string = "I Cannot do it.";
string = string.replaceAll("([Cc])annot", "\\1an not");

Desired string is "I can not do it". In Ruby '\1' replaces a string with the matched character C or c (using back reference). I don't know what to use in Java. Below is the Ruby regex which works fine:

"I Cannot do it".gsub!(/([Cc])annot/,'\1an not')
# => "I Can not do it"       
"I cannot do it".gsub!(/([Cc])annot/,'\1an not')
# => "I can not do it"