<p>Not sure if the docs are confusing, but the second parameter is the position, which is counted from end backwards when negative value. The length, the third parameter, is always counted forward (never backwards). So if you want to get the last 10 chars:</p>
<pre><code>$var(num)=$(fU{s.substr,-10,10})
</code></pre>
<p>Feel free to improve the docs.</p>

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