The Java Developers Almanac 1.4

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e440. Using the Captured Text of a Group within a Replacement Pattern

Just as it is possible to use the value of a group within the same pattern (see e439 Using the Captured Text of a Group within a Pattern), it is also possible to use a group value in the replacement pattern. Instead of a backslash, the form of the reference is $n where n is a group number starting from 0 (see e436 Capturing Text in a Group in a Regular Expression for more information on groups).

    // Compile regular expression
    String patternStr = "\\((\\w+)\\)";
    String replaceStr = "<$1>";
    Pattern pattern = Pattern.compile(patternStr);
    
    // Replace all (\w+) with <$1>
    CharSequence inputStr = "a (b c) d (ef) g";
    Matcher matcher = pattern.matcher(inputStr);
    String output = matcher.replaceAll(replaceStr);
    // a (b c) d <ef> g