function instag(tag){
var input = document.form1.texto;
//para mac
if(typeof document.selection != 'undefined' && document.selection) {
var str = document.selection.createRange().text;
input.focus();
var sel = document.selection.createRange();
sel.text = "[" + tag + "]" + str + "[/" +tag+ "]";
return;
}else if(typeof input.selectionStart != 'undefined'){
var start = input.selectionStart;
var end = input.selectionEnd;
var insText = input.value.substring(start, end);
input.value = input.value.substr(0, start) + '['+tag+']' + insText + '[/'+tag+']'+ input.value.substr(end);
return;
}else{
  input.value+=' ['+tag+']Reemplace este texto[/'+tag+']';
  return;
}
}