<textarea id="memo1" cols="25" rows="5">aaaa bbbb cccc</textarea>
<input type="button" onmousedown="add_tags('', '')" onclick="restore_selection()" value="Bold" />
<script type="text/javascript">
var ie //@cc_on = true;
var range;
var textarea = document.getElementById("memo1");
 
function add_tags(str1, str2) {
    var text, offset;
    var selection = document.selection || getSelection();
    if(ie) {
        range = selection.createRange();
        text = range.text;
        range.text = str1 + text + str2;
        offset = str2.length;
        range.moveStart("character", -text.length - offset);
        if(text.length) {
            range.moveEnd("character", -offset);
        } else {
            range.collapse();
        }
    } else {
        var selStart = textarea.selectionStart; 
        var selEnd = textarea.selectionEnd;
        text = textarea.value;
        textarea.value = text.substring(0, selStart) + str1 + text.substring(selStart, selEnd) + str2 + text.substring(selEnd, text.length);
        offset = str1.length;
        textarea.selectionStart = selStart + offset;
        textarea.selectionEnd = selEnd + offset;
    }
}
 
function restore_selection() {
    textarea.focus();
    if(ie) {
        range.select();
    }
}
</script>