(1) (?X) to define a named group NAME"
(2) \k to backref a named group "NAME"
(3) <$ to reference to captured group in matcher's replacement str
(4) group(String NAME) to return the captured input subsequence by the given "named group"

1 String pStr = "0x(?\\p{XDigit}{1,4})\\s++u\\+(?\\p{XDigit}{4})(?:\\s++)?";
2 
3 　　Matcher m = Pattern.compile(pStr).matcher(INPUTTEXT);
4 
5 　　if (m.matches()) {
6 
7 　　int bs = Integer.valueOf(m.group("bytes"), 16);
8 
9 　　int c = Integer.valueOf(m.group("char"), 16);
10 
11 　　System.out.printf("[%x] -> [%04x]%n", bs, c);
12 
13 　　}
14 
15 　　String pStr = "0x(?\\p{XDigit}{1,4})\\s++u\\+(?\\p{XDigit}{4})(?:\\s++)?";
16 
17 　　Matcher m = Pattern.compile(pStr).matcher(INPUTTEXT);
18 
19 　　if (m.matches()) {
20 
21 　　int bs = Integer.valueOf(m.group("bytes"), 16);
22 
23 　　int c = Integer.valueOf(m.group("char"), 16);
24 
25 　　System.out.printf("[%x] -> [%04x]%n", bs, c);
26 
27 　　}

1 System.out.println("0x1234 u+5678".replaceFirst(pStr, "u+$ 0x$"));