UnmatchableDollar.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
  "qhelp.dtd">
<qhelp>
<overview>
<p>
A dollar assertion <code>$</code> in a regular expression only matches at the end of the input, or
(for multi-line regular expressions) at the end of a line. If it is followed by a pattern
that must match a non-empty sequence of (non-newline) input characters, it cannot possibly match,
rendering the entire regular expression unmatchable.
</p>

</overview>
<recommendation>

<p>
Examine the regular expression to find and correct any typos.
</p>

</recommendation>
<example>
<p>
In the following example, the regular expression <code>r"\.\(\w+$\)"</code> cannot match any
string, since it contains a dollar assertion followed by an escape sequence that matches a
closing parenthesis.
</p>

<p>
The second regular expression, <code>r"\.\(\w+\)$"</code>, has the dollar at the end and will work as expected.
</p>

<sample src="UnmatchableDollar.py" />

</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/anchors.html">Start of String and End of String Anchors</a>.</li>

</references>
</qhelp>