LlInsertString

Returns a string dst with src inserted starting at pos.

pos does not support negative indexes.
i.e. unlike other somewhat similar string functions such as llGetSubString and llDeleteSubString, you cannot use -1 for the counting with this function. You may use instead the function provided a bit further below.

llInsertString("input", 2, "put out")// returns "input output"

This function will allow you to use a negative number as the position:

string insertString(string destination, integer position, string str) {
    if (position < 0)
        if ((position += llStringLength(destination)) < 0)
            position = 0;
    return llInsertString(destination, position, str);
}

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