Validating alphabetic only string in javascript
<p>How can I quickly validate if a string is alphabetic only, e.g</p>

<pre><code>var str = "!";
alert(isLetter(str)); // false

var str = "a";
alert(isLetter(str)); // true
</code></pre>

<p>Edit : I would like to add parenthesis i.e <code>()</code> to an exception, so </p>

<pre><code>var str = "(";
</code></pre>

<p>or </p>

<pre><code>var str = ")";
</code></pre>

<p>should also return true.</p>
