Use Java and RegEx to convert casing in a string

You can’t do this in Java regex. You’d have to manually post-process using String.toUpperCase() and toLowerCase() instead.

Here’s an example of how you use regex to find and capitalize words of length at least 3 in a sentence

    String text = "no way oh my god it cannot be";
    Matcher m = Pattern.compile("\\b\\w{3,}\\b").matcher(text);

    StringBuilder sb = new StringBuilder();
    int last = 0;
    while (m.find()) {
        sb.append(text.substring(last, m.start()));
        last = m.end();

    // prints "no WAY oh my GOD it CANNOT be"

Note on appendReplacement and appendTail

Note that the above solution uses substring and manages a tail index, etc. In fact, you can go without these if you use Matcher.appendReplacement and appendTail.

    StringBuffer sb = new StringBuffer();
    while (m.find()) {

Note how sb is now a StringBuffer instead of StringBuilder. Until Matcher provides StringBuilder overloads, you’re stuck with the slower StringBuffer if you want to use these methods.

It’s up to you whether the trade-off in less efficiency for higher readability is worth it or not.

See also

  • StringBuilder and StringBuffer in Java

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