Well, replacement and pattern are two different strings.
String replacement = "AAA mmmm.nnn?ooo=45302 EEEEEE FFFFF GGGGGG";
String pattern = "KLDKFVN FDFJDVmmmm.nnn?ooo=9876ERF
However, they both have a similar pattern which is mmmm.nnn?ooo equal to a number. My question is asking how to get the number (9876) from the string pattern and replace the number (45302) of the string replacement i.e. to change the string replacement number (45302) to 9876. Then the new string should be AAA mmmm.nnn?ooo=9876 EEEEEE FFFFF GGGGGG.
The regex = "mmmm.nnn\\?ooo=[0-9]+" can get the first step right to yield mmmm.nnn?ooo=9876 from the string pattern. The next step is we need to do substitution. How can we replace the mmmm.nnn?ooo=45302 from the string replacement with mmmm.nnn?ooo=9876 in order to have a new string AAA mmmm.nnn?ooo=9876 EEEEEE FFFFF GGGGGG.
Hope my question is more clear for you :)
More detail can be found from the link provided below.