UnmatchableCaret.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
  "qhelp.dtd">
<qhelp>
<overview>
<p>
The caret character <code>^</code> anchors a regular expression to the beginning of the input, or
(for multi-line regular expressions) to the beginning of a line.
If it is preceded by a pattern that must match a non-empty sequence of (non-newline) input characters,
then the entire regular expression cannot match anything.
</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"\[^.]*\.css"</code> cannot match any
string, since it contains a caret assertion preceded by an escape sequence that matches an
opening bracket.
</p>
<p>
In the second regular expression, <code>r"[^.]*\.css"</code>, the caret is part of a character class, and will not match the start of the string.
</p>

<sample src="UnmatchableCaret.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>