Putting char into a java string for each N characters

Try:

String s = // long string
s.replaceAll("(.{10})", "$1<br>");

EDIT: The above works... most of the time. I've been playing around with it and came across a problem: since it constructs a default Pattern internally it halts on newlines. to get around this you have to write it differently.

public static String insert(String text, String insert, int period) {
    Pattern p = Pattern.compile("(.{" + period + "})", Pattern.DOTALL);
    Matcher m = p.matcher(text);
    return m.replaceAll("$1" + insert);
}

and the astute reader will pick up on another problem: you have to escape regex special characters (like "$1") in the replacement text or you'll get unpredictable results.

I also got curious and benchmarked this version against Jon's above. This one is slower by an order of magnitude (1000 replacements on a 60k file took 4.5 seconds with this, 400ms with his). Of the 4.5 seconds, only about 0.7 seconds was actually constructing the Pattern. Most of it was on the matching/replacement so it doesn't even ledn itself to that kind of optimization.

I normally prefer the less wordy solutions to things. After all, more code = more potential bugs. But in this case I must concede that Jon's version--which is really the naive implementation (I mean that in a good way)--is significantly better.

Something like:

public static String insertPeriodically(
    String text, String insert, int period)
{
    StringBuilder builder = new StringBuilder(
         text.length() + insert.length() * (text.length()/period)+1);

    int index = 0;
    String prefix = "";
    while (index < text.length())
    {
        // Don't put the insert in the very first iteration.
        // This is easier than appending it *after* each substring
        builder.append(prefix);
        prefix = insert;
        builder.append(text.substring(index, 
            Math.min(index + period, text.length())));
        index += period;
    }
    return builder.toString();
}

I got here from String Manipulation insert a character every 4th character and was looking for a solution on Android with Kotlin.

Just adding a way to do that with Kotlin (you got to love the simplicity of it)

val original = "123498761234"
val dashed = original.chunked(4).joinToString("-") // 1234-9876-1234