How to write a regular expression
<p>I need to write a regular expression that will be correct for telephone number with some rules:</p>

<pre><code>1) max length 13 symbols
2) it should start from '+'
3) it should contain sonly numbers [0-9]
</code></pre>

<p>So, for example, it should be like this:</p>

<pre><code>+447289347598342745
</code></pre>
