</p>

<h3 id="Letters_and_digits">Letters and digits</h3>

<p>
The underscore character <code>_</code> (U+005F) is considered a lowercase letter.
</p>
<pre class="ebnf">
letter        = unicode_letter | "_" .
decimal_digit = "0" … "9" .
binary_digit  = "0" | "1" .
octal_digit   = "0" … "7" .
hex_digit     = "0" … "9" | "A" … "F" | "a" … "f" .
</pre>