正则表达式替换替换字符串

Question

Let's consider the following example:

String s = str.replaceAll("regexp", "$1");

Some languages allow us to specify \\U$1 in place of $1 which converts matched groups with uppercase letters. How can I achieve the same using Java?

I know we can use Pattern class and get the group and convert it to uppercase, but that's not what I am looking for. I want to just change $1 with something that gets the job done.

I have also tried:

String s = str.replaceAll("regexp", "$1".toUpperCase());

But it looks like "$1".toUpperCase() is "$1" and not the match. I confirmed it using:

String s = str.replaceAll("regexp", method("$1"));

// method declared as method()
private static String method(String s) {
    System.out.println(s); // prints "$1"
    return s;
}

Is it even allowed in Java?

EDIT:

String s = "abc";
System.out.println(s.replaceAll("(a)", "$1")); // should print "Abc"

EDIT FOR POSSIBLE DUPE:

I am not looking for way using m.group() , is it possible using something like \\U$1 in place of $1 with replaceAll()

Answer 1

\\\\U is not implemented in the java regex AFAIK and you can't do it with a regex as such ( .NET has it IIRC). It's a bit verbose, but one way to do it would be:

    String test = "abc";
    Pattern p = Pattern.compile("(a)");
    Matcher m = p.matcher(test);

    StringBuilder sb = new StringBuilder();
    if (m.find()) {
        String match = test.substring(m.start(1), m.end(1));
        m.appendReplacement(sb, match.toUpperCase());
    }

    m.appendTail(sb);
    System.out.println(sb.toString()); 

Answer 2

Since Java 9, we can provide a Function to Matcher#replaceAll(Function<MatchResult,​String> replacer) . It is more concise than other answers here. Eg:

Pattern.compile("regexp")
       .matcher(str)
       .replaceAll(mr -> mr.group().toUpperCase());

We can fully customize this behaviour since we have a hold on MatchResult :

Pattern.compile("regexp")
       .matcher(str)
       .replaceAll(mr -> {
                String.format("%s %s", 
                              mr.group(1).toUpperCase),
                              mr.group(2).indent(4);
                   });

Answer 3

I think you should consider StringUtils from Apache Commons.

This is an example:

String s = "abcad";

String replacer = "a";

System.out.println(StringUtils.replaceChars(s, replacer, replacer.toUpperCase()));//<--AbcAd

Pls also consider this avoids you to implement the algorythm that necessarily will be under the hood, and the fact the every jar library introduced in a project is basically a new weak point.

This is maven dependency:

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.7</version>
</dependency>

Hope it helps.