This is an off-by-one error. The first two parameters to replace(...) are the starting index of the substring to replace, inclusive, and the ending of the substring to replace, exclusive. By using the same value for both, you are in effect "replacing" the nothing between indexes 1 and 2. This inclusive/exclusive way of expressing a range is extremely common.

There is no such method on StringBuffer. Perhaps the following will help:

StringBuffer str1 = new StringBuffer("whatever");
// to get a String result:
String str2 = str1.toString().replaceAll(regex, substr);
// to get a StringBuffer result:
StringBuffer str3 = new StringBuffer(str2);

The Javadocs explicitly describe the effect as "First the characters in the substring [actually, subsequence] are removed". That means that the characters from index 3 to index 19 in the original sequence (all blanks) are removed, leaving the first three characters from your example on one end and the last ten on the other. The docs say nothing about taking a substring of the replacement string, as you surmised. Indeed, they explicitly state that "then the specified String is inserted at start." The whole replacement String, not just the part that would have fit. So now you have three blanks plus the thirty-character replacement plus ten blanks in the result.

"If no such character exists" means if end is larger than the original sequence length, no error occurs, just the whole rest of the original sequence is removed. So if your original sequence had been of length ten, then removing from index 3 inclusive to 20 exclusive would remove all the sequence's characters from index 3 on, before replacement.

Well, you can write a loop:

public static void replaceAll(StringBuilder builder, String from, String to) {
    int index = builder.indexOf(from);
    while (index != -1) {
        builder.replace(index, index + from.length(), to);
        index += to.length(); // Move to the end of the replacement
        index = builder.indexOf(from, index);
    }
}

Note that in some cases it may be faster to use lastIndexOf, working from the back. I suspect that's the case if you're replacing a long string with a short one - so when you get to the start, any replacements have less to copy. Anyway, this should give you a starting point.