public static String patternLowerCase(String pattern, String string) {
        if (string == null){
            return null;
        }
        Matcher m = Pattern.compile(pattern).matcher(string);
 
        StringBuffer sb = new StringBuffer();
        while (m.find()) {
            m.appendReplacement(sb, m.group().toLowerCase());
        }
        m.appendTail(sb);
 
     return sb.toString();
    }
    public static String patternUpperCase(String pattern, String string) {
        if (string == null){
            return null;
        }
        Matcher m = Pattern.compile(pattern).matcher(string);
 
        StringBuffer sb = new StringBuffer();
        while (m.find()) {
            m.appendReplacement(sb, m.group().toUpperCase());
        }
        m.appendTail(sb);
 
     return sb.toString();
    }

Example of use:

//Here's an example of how you use regex to find and capitalize words of length at least 3 in a sentence 
String text = "example of how you use";
String pattern = "\b\w{3,}\b";
String res = patternUpperCase(pattern, text);
System.out.println(res);
// prints "EXAMPLE of HOW YOU USE"