A DESCRIPTION OF THE REQUEST :
In many applications, StringBuilder is used over StringBuffer as unless synchronization is required, StringBuilder will generally be faster. Matcher, a commonly used class, has many methods related to the usage of a StringBuffer, and does not provide StringBuilder-based alternatives.

In my opinion, methods should be added to allow the usage of a StringBuilder. Ideally, these shouldn't just create a StringBuffer object with the same contents as the StringBuilder, but instead be properly optimized for usage with StringBuilder.

Quick Links:

Matcher: http://docs.oracle.com/javase/7/docs/api/java/util/regex/Matcher.html
StringBuffer: http://docs.oracle.com/javase/7/docs/api/java/lang/StringBuffer.html
StringBuilder: http://docs.oracle.com/javase/7/docs/api/java/lang/StringBuilder.html

JUSTIFICATION :
While not a massive bug, this issue is something which wouldn't be incredibly difficult to implement and would generally help improve performance in areas where looped String concatenation is required along with the usage of a Matcher(s). This is not a particularly obscure situation.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
I would like to see methods in java.util.Matcher which take a StringBuffer become compatible with StringBuilders. Note that this is not a request to just add methods which convert a StringBuilder into a new StringBuffer, but one to add properly optimized methods for StringBuilder.
ACTUAL -
Currently, there are no methods which allow the usage of StringBuilder.

---------- BEGIN SOURCE ----------
Pattern regex = Pattern.compile("^\\s*((-?[0-9]?\\.?[0-9]+\\s*){3})$");
// StringBuilder would be optimal
StringBuffer sb = new StringBuffer();
for (String line : listOfStrings) {
    Matcher m = regex.matcher(line);
    if (m.find()) {
        m.appendReplacement(sb, "v $1");
        m.appendTail(sb);
        // do something with it (in the actual application this is outputted to a file)
        sb.setLength(0);
    }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
As above, a StringBuffer can be used, but is not optimal.