Most vi (and therefore vim) features were derived from ed.  vi and ed both predate perl by at least a decade or two.  A better question might be &quot;why doesn&#39;t Perl use the same regex syntax as vi?&quot;. 

Of course, one could also argue that the kinds of regular expressions that one would wish to write inside a text editor to perform common tasks are probably rather different to those you might wish to write inside a programming language.

There is a plugin called [eregex.vim][1] which translates from PCRE to Vim&#39;s syntax. It takes [over a thousand lines of vim to achieve that translation][2]!


  [1]: http://www.vim.org/scripts/script.php?script_id=3282
  [2]: https://github.com/othree/eregex.vim/blob/master/plugin/eregex.vim

In addition to the mentioned reasons vim has some cards in the sleeve when it comes to regex, for example, to match positive look behind we do:

    :%s/^foo \zsbar/moo/g

The above command will substitute &quot;bar&quot; for &quot;moo&quot; in the lines started with &quot;foo&quot;.

the `\zs` makes easier to set a positive look-behind and `\ze` makes easier to make a positive look-ahead.

We also have a way to match only in the &quot;visual area&quot; -&gt;  \%V

    :&#39;&lt;,&#39;&gt;s/\%Vthis/that/g

Although using the global flag &quot;g&quot; the substitution is restricted to the visual are due `\%V`

To read an awesome article about how amazing vim regexes can be, follow this link: https://bitbucket.org/snippets/sergio/7nzqxx

We can also use some &quot;submatch&quot; tricks on vim substitution
https://dev.to/voyeg3r/vim-math-on-vim-substitution-4obo

I have found several topics with this title, but none of their solutions worked for me.  I have two Django sites running on my server, both through Apache using different virtualhosts on two ports fed by my Nginx frontend (using for static files).  One site uses MySql and runs just fine.  The other uses Sqlite3 and gets the error in the title.

   I downloaded a copy of sqlite.exe and looked at the mysite.sqlite3 (SQLite database in this directory) file and there is indeed a django_session table with valid data in it.  I have the sqlite.exe in my system32 as well as the site-packages folder in my Python path.

   Here is a section of my settings.py file:

    MANAGERS = ADMINS
    
    DATABASES = {
        &#39;default&#39;: {
            &#39;ENGINE&#39;: &#39;django.db.backends.sqlite3&#39;, # Add &#39;postgresql_psycopg2&#39;, &#39;postgresql&#39;, &#39;mysql&#39;, &#39;sqlite3&#39; or &#39;oracle&#39;.
            &#39;NAME&#39;: &#39;mysite.sqlite3&#39;,         # Or path to database file if using sqlite3.
            &#39;USER&#39;: &#39;&#39;,                      # Not used with sqlite3.
            &#39;PASSWORD&#39;: &#39;&#39;,                  # Not used with sqlite3.
            &#39;HOST&#39;: &#39;&#39;,                      # Set to empty string for localhost. Not used with sqlite3.
            &#39;PORT&#39;: &#39;&#39;,                      # Set to empty string for default. Not used with sqlite3.
        }
    }

I did use the python manage.py syncdb with no errors and just a &quot;No Fixtures&quot; comment.

  Does anyone have any ideas what else might be going on here?  I&#39;m considering just transferring everything over to my old pal MySql and just ignoring Sqlite, as really it&#39;s always given me some kind of trouble.  I was only using it for the benefit of knowing it anyway.  I have no overwhelming reason why I should use it.  But again, just for my edification does anyone know what this problem is?  I don&#39;t like to give up.

Django: no such table: django_session

I have a package `mypack` with modules `mod_a` and `mod_b` in it. I intend the package itself and `mod_a` to be imported freely:

    import mypack
    import mypack.mod_a

However, I&#39;d like to keep `mod_b` for the exclusive use of `mypack`. That&#39;s because it exists merely to organize the latter&#39;s internal code. 

My first question is, is it an accepted practice in Python programming to have &#39;private&#39; modules like this?

If yes, my second question is, what is the best way to convey this intention to the client? Do I prefix the name with an underscore (i.e. `_mod_b`)? Or would it be a good idea to declare a sub-package `private` and place all such modules there?

Python: &#39;Private&#39; module in a package

Why do you have to make your regex &quot;very magic&quot; so that you don&#39;t have to escape your capture quotes? And if you set your environment to very magic, you are non-standard and may have compliance issues.  I am just wondering why vim uses a different regex syntax than say, perl? 

Why does VIM have its own regex syntax?

<p>Why do you have to make your regex "very magic" so that you don't have to escape your capture quotes? And if you set your environment to very magic, you are non-standard and may have compliance issues.  I am just wondering why vim uses a different regex syntax than say, perl?</p>


Have I missed a standard API call that removes trailing insignificant zeros from a number?


    var x = 1.234000; // to become 1.234
    var y = 1.234001; // stays 1.234001

`Number.toFixed()` and `Number.toPrecision()` are not quite what I&#39;m looking for.

Remove insignificant trailing zeros from a number?

How can I write a regex that matches only letters?

Regex to match only letters

Even after years of programming, I&#39;m ashamed to say that I&#39;ve never really fully grasped regular expressions. In general, when a problem calls for a regex, I can usually (after a bunch of referring to syntax) come up with an appropriate one, but it&#39;s a technique that I find myself using increasingly often. 

So, to teach myself and understand regular expressions _properly_, I&#39;ve decided to do what I always do when trying to learn something; i.e., try to write something ambitious that I&#39;ll probably abandon as soon as I feel I&#39;ve learnt enough.

To this end, I want to write a regular expression parser in Python. In this case, &quot;learn enough&quot; means that I want to implement a parser that can understand Perl&#39;s extended regex syntax completely. However, it doesn&#39;t have to be the most efficient parser or even necessarily usable in the real-world. It merely has to correctly match or fail to match a pattern in a string.

The question is, where do I start? I know almost nothing about how regexes are parsed and interpreted apart from the fact that it involves a finite state automaton in some way. Any suggestions for how to approach this rather daunting problem would be much appreciated.

**EDIT:** I should clarify that while I&#39;m going to _implement_ the regex parser in Python, I&#39;m not overly fussed about what programming language the examples or articles are written in. As long as it&#39;s not in Brainfuck, I will probably understand enough of it to make it worth my while.

Writing a parser for regular expressions

I want to filter strings in a list based on a regular expression.

Is there something better than `[x for x in list if r.match(x)]` ?

Regular Expressions: Search in list

&gt; &lt;sub&gt;_This is the second part of a series of educational regex articles. It shows how lookaheads and nested references can be used to match the non-regular languge a&lt;sup&gt;n&lt;/sup&gt;b&lt;sup&gt;n&lt;/sup&gt;. Nested references are first introduced in: [How does this regex find triangular numbers?][p1]_&lt;/sub&gt;

One of the archetypal non-[regular languages][wrl] is:

&gt; `L = { a`&lt;sup&gt;n&lt;/sup&gt;`b`&lt;sup&gt;n&lt;/sup&gt;` : n &gt; 0 }`  

This is the language of all non-empty strings consisting of some number of `a`&#39;s followed by an equal number of `b`&#39;s. Examples of strings in this language are `ab`, `aabb`, `aaabbb`.

This language can be show to be non-regular by the [pumping lemma][wpl]. It is in fact an archetypal [context-free language][wcfl], which can be generated by the [context-free grammar][wcfl] `S → aSb | ab`.

Nonetheless, modern day regex implementations clearly recognize more than just regular languages. That is, they are not &quot;regular&quot; by formal language theory definition. PCRE and Perl supports recursive regex, and .NET supports balancing groups definition. Even less &quot;fancy&quot; features, e.g. backreference matching, means that regex is not regular.

But just how powerful is this &quot;basic&quot; features? Can we recognize `L` with Java regex, for example? Can we perhaps combine lookarounds and nested references and have a pattern that works with e.g. [`String.matches`](http://download.oracle.com/javase/6/docs/api/java/lang/String.html#matches%28java.lang.String%29) to match strings like `ab`, `aabb`, `aaabbb`, etc?

### References

* [perlfaq6: Can I use Perl regular expressions to match balanced text?](http://perldoc.perl.org/perlfaq6.html#Can-I-use-Perl-regular-expressions-to-match-balanced-text?)
* [MSDN - Regular Expression Language Elements - Balancing Group Definitions](http://msdn.microsoft.com/en-us/library/bs2twtah.aspx#balancing_group_definition)
* [pcre.org - PCRE man page](http://www.pcre.org/pcre.txt)
* [regular-expressions.info](http://www.regular-expressions.info/) - [Lookarounds](http://www.regular-expressions.info/lookaround.html) and [Grouping and Backreferences](http://www.regular-expressions.info/brackets.html)
* [`java.util.regex.Pattern`](http://java.sun.com/javase/6/docs/api/java/util/regex/Pattern.html)

### Linked questions

* [Does lookaround affect which languages can be matched by regular expressions?](https://stackoverflow.com/questions/2974210/does-lookaround-affect-which-languages-can-be-matched-by-regular-expressions)
* [.NET Regex Balancing Groups vs PCRE Recursive Patterns](https://stackoverflow.com/questions/3349999/net-regex-recursive-patterns)

[p1]: https://stackoverflow.com/questions/3627681/how-does-this-regex-find-triangular-numbers/
[wrl]: http://en.wikipedia.org/wiki/Regular_language
[wpl]: http://en.wikipedia.org/wiki/Pumping_lemma_for_regular_languages
[wcfg]: http://en.wikipedia.org/wiki/Context-free_grammar
[wcfl]: http://en.wikipedia.org/wiki/Context-free_language
[wpcre]: http://en.wikipedia.org/wiki/Perl_Compatible_Regular_Expressions

How can we match a^n b^n?

Is there a command to delete a line (or several lines) that is immediately below current line?
Currently I&#39;m doing it as:
&lt;kbd&gt;j&lt;/kbd&gt;&lt;kbd&gt;dd&lt;/kbd&gt; and then &lt;kbd&gt;.&lt;/kbd&gt; to repeat as needed.
Is there a command that would combine all these?

UPDATE: The reason I would like to have such command is that I don&#39;t like to move away from current position, yet be able to delete lines below.

How to delete line(s) below current line in vim?

I love vim, but one common gotcha is:

 - yank a line
 - go to where you would like to paste it
 - delete what&#39;s there
 - paste your yank, only to discover that it pastes what you just deleted

Obviously the workflow is delete first, yank second.  But it would be reeeeeaaaaaalllly nice if I didn&#39;t *have* to.  Anyone have a trick for this? Does vim have a paste buffer that works well, or is there a .vimrc setting I can change?

Any way to delete in vim without overwriting your last yank?

I prefer to use `tab` than `white space`(may be a little different from most of others)

But I found, when I hit `Enter` at the end of line, it will add some **white spaces**, but not **tab**. So, I have to delete them and press **tab**.

I want to know how to set vim as:

1. use only **tab** to indent the lines
2. a tab looks like 4-spaces, but actually is a tab
3. when hit `enter` at the end of a line, the new line is started with only tabs

I&#39;ve googled for this for a while, but not found a good answer. Thank you in advance

-------

**UPDATE**

The answer @Alok has provided works well in most of cases. But I just found, sometimes, it depends on the file type. For example, if you are editing a `haml` file, and there is a `haml.vim` in your `vimfiles/indent/`, then all the tabs will be converted to `space`. So if you want it to be `tab` only, you should modify(or delete) the corresponding indent file.

How to use only tab (not space) in vim

I&#39;ve found several answers on how to indent multiple lines in vim, I want to know how to take a single line and indent it more than once.  In effect, I want a shorter version of the following command:
&quot;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;&quot;
(That is 10 right bracket &quot;greater-than&quot; signs.)

How do I indent a single line multiple times in vi or vim?

I setup `git diff` to wrap into vimdiff, using &quot;[Git Diff with Vimdiff][1]&quot; as a guide, and it&#39;s working as expected unless there are many files with changes. 

When there are multiple files with changes and I run `git diff`, it opens the first file and, after quitting the first instance of vimdiff, I&#39;m presented with the following message:

    external diff died, stopping at filename

This is a completely different behavior than I am used to.  I had a similar setup in the past with SVN and, when diffing against multiple files, I would review the first file, then write and quit using `:wq` and the next file with differences would open up.

This is not the case with Git. I tried `:n[ext]`, but doing so does not fill the left window with the original file so that it can be diffed against the modified version.
         

  [1]: http://technotales.wordpress.com/2009/05/17/git-diff-with-vimdiff/

Content Type	Original Author	Original Content on Stackoverflow
Question	user210757	View Question on Stackoverflow
Solution 1 - Regex	Gian	View Answer on Stackoverflow
Solution 2 - Regex	Evgeni Sergeev	View Answer on Stackoverflow
Solution 3 - Regex	SergioAraujo	View Answer on Stackoverflow

Why does VIM have its own regex syntax?

Regex Problem Overview

Regex Solutions

Solution 1 - Regex

Solution 2 - Regex

Solution 3 - Regex

Python: 'Private' module in a package

Django: no such table: django_session

Attributions