<div>
:? means: 0 or more &quot;:&quot;<br />
<br />
The first (:?) is the capturing group 2. The second :? is not in a capturing group.<br />
<br />
The only reason I can think of for (?!\\) is Windows UNC paths, that contain \.<br />
<br />
So the regular expression will stop if that path is of the type: user:pass\\hostname<br />
<br />
HTH,<br />
Dan
</div>


<div>
Thank you but i tried with windows unc with/without negative look-ahead but same result it's not working w that re string.
</div>


<div>
<blockquote>
:? means: 0 or more &quot;:&quot;
</blockquote>
No, it means zero or one.<br />
<br />
The negative lookahead is pointless. The very next character to be found is an @, so the lookahead will always be true.
</div>


<div>
Selected answer is not correct. If the lookahead were instead a lookbehind, then Dan Craciun would be correct.<br />
<br />
You can see why using the following string:<br />
<br />

<div class="indent">
user:p@ssword4\@host@2:11
</div>
<br />
By Dan's (and the referenced article's logic), the password should be &quot;p&quot;, but the result is &quot;p@ssword4\\@host&quot;. As I said, the look<b>ahead</b> is pointless in this scenario.
</div>


<div>
thank you! I reached out author of that code and he said the same thing!
</div>


<div>
<div class="content wysiwyg-content">
Hi, below python regular expression is for getting id/pwd/hostname/port from user input.<br />
<br />
Python Re:<br />
(?P&lt;username&gt;[^@:]*)<b>(:?)</b>(?P&lt;password&gt;.*)<b>(?!\\)</b>@(?P&lt;hostname&gt;[^:]*)<b>:?</b>(?P&lt;port&gt;[0-9]*)<br />
<br />
Target String:<br />
aaa:bbb@hostname:22<br />
<br />
Could you please explain above bold formatted characters' roles ?<br />
I am not sure why <b>(:?)</b> has parenthesis however <b>:?</b> before &quot;port&quot; parsing does not have one.<br />
I am not sure why <b>(?!\\)</b> was used why do we need negative look-ahead for '\' ? It works without <b>(?!\\)</b> regex. <br />
<br />
<br />
Regular Expression tool: <a href="http://pythex.org/" rel="ugc">http://pythex.org/</a><br />
Code used: <a href="https://github.com/pexpect/pexpect/blob/master/examples/hive.py" rel="ugc">https://github.com/pexpect/pexpect/blob/master/examples/hive.py</a>
</div>
</div>


Hi, below python regular expression is for getting id/pwd/hostname/port from user input.

Python Re:
(?P<username>[^@:]*)(:?)(?P<password>.*)(?!\\)@(?P<hostname>[^:]*):?(?P<port>[0-9]*)

Target String:
aaa:bbb@hostname:22

Could you please explain above bold formatted characters' roles ?
I am not sure why (:?) has parenthesis however :? before "port" parsing does not have one.
I am not sure why (?!\\) was used why do we need negative look-ahead for '\' ? It works without (?!\\) regex. 


Regular Expression tool: http://pythex.org/
Code used: https://github.com/pexpect/pexpect/blob/master/examples/hive.py

Python negative lookahead for aaa:bbb@hostname

A regular expression ("regex") is a sequence of characters that define a search pattern, mainly for use in pattern matching with strings, or string matching, i.e. "find and replace"-like operations. Regular expression processors are found in several search engines, search and replace dialogs of several word processors and text editors, and in the command lines of text processing utilities, such as sed and AWK. Many programming languages provide regular expression capabilities, some built-in, for example Perl, JavaScript, Ruby, AWK, and Tcl, and others via a standard library, for example .NET languages, Java, Python and C++ (since C++11). Most other languages offer regular expressions via a library.

Regular Expressions

Python is a widely used general-purpose, high-level programming language. Its design philosophy emphasizes code readability, and its syntax allows programmers to express concepts in fewer lines of code than would be possible in other languages. Python supports multiple programming paradigms, including object-oriented, imperative and functional programming or procedural styles. It features a dynamic type system and automatic memory management and has a large and comprehensive set of standard libraries, including NumPy, SciPy, Django, PyQuery, and PyLibrary.