Исходный текст<br>
 aaaaaaaa:bbbb!cbbb sdfsdfs     sdf   aaaaaa aa:yb@#%$%bbbbbaaaaaaaa:ybbbbbb aaaaaaaa:ubbbb@%^bb aaaaaaaa:ibbbbbb Привет Мир!Я делаю скрипт....         aaaaaaa:ibbbbbb 
<br>
<script>
function redhash(text) {
var re = /:([^ \f\n\r\t\v:]+)\s/mig;
var str = text;
document.write(str.replace(re,':<b>$1</b> '));
}


var r = " aaaaaaaa:bbbb!cbbb sdfsdfs     sdf   aaa#$aaa aWR$%a:bbbbbaaaaaaaa:ybbbbbb $aaa$aaaaa:ubbbb@%^bb aaaaaaaa:ibbbbbb Привет Мир!Я делаю скрипт....         aaaaaaa:ibbbbbb ";
</script>
Готовый текст<br>
<script>
redhash(r) ;
</script>