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

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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"

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class="normal">Answer

What
about

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