textarea name=start cols=50 rows=10textarea
textarea name=out cols=50 rows=10textarea
script type=textjavascript
var s = document.getElementsByName('start')[0],o = document.getElementsByName('out')[0];
s.onkeyup =  s.onkeypress = function ()
{ var text = this.value.replace(^(.)$gm,@$ (Privet)) 
  o.value = text
}script

<textarea name="start" cols="50" rows="10"></textarea>
<textarea name="out" cols="50" rows="10"></textarea>
<script type="text/javascript">
var s = document.getElementsByName('start')[0],o = document.getElementsByName('out')[0];
s.onkeyup =  s.onkeypress = function ()
{ var text = this.value.replace(/^(.*?)$/gm,"111$1222") 
  o.value = text
}</script>

<textarea name="start" cols="50" rows="10"></textarea>
<textarea name="out" cols="50" rows="10"></textarea>
<script type="text/javascript">
var s = document.getElementsByName('start')[0],o = document.getElementsByName('out')[0];
s.onkeyup = s.onkeypress = function ()
{ var text = "111"+this.value+"222"
o.value = text
}</script>