DuplicateCharacterInSet.qhelp

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

<overview>
<p>
Character classes in regular expressions represent sets of characters, so there is no need to specify
the same character twice in one character class. Duplicate characters in character classes are at best
useless, and may even indicate a latent bug.
</p>

</overview>
<recommendation>

<p>Determine whether a character is simply duplicated or whether the character class was in fact meant as a group.
If it is just a duplicate, then remove the duplicate character.
If was supposed to be a group, then replace the square brackets with parentheses.
</p>


</recommendation>
<example>
<p>
In the following example, the character class <code>[password|pwd]</code> contains two instances each
of the characters <code>d</code>, <code>p</code>, <code>s</code>, and <code>w</code>. The programmer most likely meant
to write <code>(password|pwd)</code> (a pattern that matches either the string <code>"password"</code>
or the string <code>"pwd"</code>), and accidentally mistyped the enclosing brackets.
</p>

<sample src="DuplicateCharacterInSet.py" />

<p>
To fix this problem, the regular expression should be rewritten to <code>r"(password|pwd)"</code>.
</p>

</example>
<references>

<li>Python Standard Library: <a href="https://docs.python.org/library/re.html">Regular expression operations</a>.</li>
<li>Regular-Expressions.info: <a href="http://www.regular-expressions.info/charclass.html">Character Classes or Character Sets</a>.</li>

</references>
</qhelp>