Trim String in Java while preserve full word

Here is a simple, regex-based, 1-line solution:

str.replaceAll("(?<=.{12})\\b.*", "..."); // How easy was that!? :)

Explanation:

• (?<=.{12}) is a negative look behind, which asserts that there are at least 12 characters to the left of the match, but it is a non-capturing (ie zero-width) match
• \b.* matches the first word boundary (after at least 12 characters - above) to the end

This is replaced with "..."

Here's a test:

public static void main(String[] args) {
    String input = "The quick brown fox jumps over the lazy dog.";
    String trimmed = input.replaceAll("(?<=.{12})\\b.*", "...");
    System.out.println(trimmed);
}

Output:

The quick brown...

---

If performance is an issue, pre-compile the regex for an approximately 5x speed up (YMMV) by compiling it once:

static Pattern pattern = Pattern.compile("(?<=.{12})\\b.*");

and reusing it:

String trimmed = pattern.matcher(input).replaceAll("...");

Below is a method I use to trim long strings in my webapps. The "soft" boolean as you put it, if set to true will preserve the last word. This is the most concise way of doing it that I could come up with that uses a StringBuffer which is a lot more efficient than recreating a string which is immutable.

public static String trimString(String string, int length, boolean soft) {
    if(string == null || string.trim().isEmpty()){
        return string;
    }

    StringBuffer sb = new StringBuffer(string);
    int actualLength = length - 3;
    if(sb.length() > actualLength){
        // -3 because we add 3 dots at the end. Returned string length has to be length including the dots.
        if(!soft)
            return escapeHtml(sb.insert(actualLength, "...").substring(0, actualLength+3));
        else {
            int endIndex = sb.indexOf(" ",actualLength);
            return escapeHtml(sb.insert(endIndex,"...").substring(0, endIndex+3));
        }
    }
    return string;
}

Update

I've changed the code so that the ... is appended in the StringBuffer, this is to prevent needless creations of String implicitly which is slow and wasteful.

Note: escapeHtml is a static import from apache commons:

import static org.apache.commons.lang.StringEscapeUtils.escapeHtml;

You can remove it and the code should work the same.