EmptyCharacterClass.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
  "qhelp.dtd">
<qhelp>

<overview>
<p>
An empty character class in a regular expression does not match anything and may indicate missing code.
</p>

</overview>
<recommendation>

<p>Omit the empty character class. If the whole regular expression would become empty, use
<code>/(?:)/</code> to express a deliberately empty regular expression.</p>

</recommendation>
<example>
<p>
In the following example, the programmer presumably meant to write a regular expression that matches
an opening square bracket or curly brace, followed by one or more letters or digits, followed by
a closing square bracket or curly brace. However, they forgot to escape the closing square bracket
with a backslash, leading to an empty character class. The resulting regular expression is malformed
and could be interpreted differently on different platforms.
</p>

<sample src="examples/EmptyCharacterClass.js" />

<p>
To fix this problem, the regular expression should be rewritten to <code>/[[{]\w+[\]}]/</code>.
</p>

</example>
<references>

<li>Mozilla Developer Network: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions">JavaScript Regular Expressions</a>.</li>

</references>
</qhelp>