function add_tag(stag, etag) {
    obj = document.form.text;
    r = document.selection.createRange();
    
    if(document.selection.type == 'Text' && (obj.value.indexOf(r.text) != -1)) {
  a = r.text;
  r.text = stag + r.text + etag;
  if (obj.value.indexOf(document.selection.createRange().text) == -1) {
     document.selection.createRange().text = a;
  }
    }
    else {
  insert_tag(obj, stag + etag);
    }
    
    obj.focus();
}

function insert_tag(textel, text) {
    if (textel.createTextRange && textel.caretPos) {
  var caretPos = textel.caretPos;
  caretPos.text = caretPos.text.charAt(caretPos.text.length - 1) == ' ' ? text + ' ' : text;
    }
    else {
  textel.value  =  textel.value + text;
    }

    textel.focus();
}

[HTML] pobierz, plaintext 
<input type="button" class="style3"  style="background-color: white; padding-left: 3px; padding-right: 3px; font-weight: bold; cursor: hand" onClick="insert_tag("[b]","[/b]")" value="B">
[HTML] pobierz, plaintext