In a character class (square brackets) any character except `^`, `-`, `]` or `\` is a literal.

This website is a brilliant reference and has lots of info on the nuances of different regex flavours.
http://www.regular-expressions.info/refcharclass.html

I&#39;m working in (formerly Twitter) Bootstrap 2 and I wanted to style buttons as though they were normal links. Not just any normal links, though; these are going in a `&lt;ul class=&quot;nav nav-tabs nav-stacked&quot;&gt;` container. The markup will end up like this:

    &lt;form action=&quot;...&quot; method=&quot;post&quot;&gt;
      &lt;div class=&quot;row-fluid&quot;&gt;
        &lt;!-- Navigation for the form --&gt;
        &lt;div class=&quot;span3&quot;&gt;
          &lt;ul class=&quot;nav nav-tabs nav-stacked&quot;&gt;
            &lt;li&gt;&lt;button type=&quot;submit&quot; name=&quot;op&quot; value=&quot;Link 1&quot;&gt;Link 1&lt;/button&gt;&lt;/li&gt;
            &lt;li&gt;&lt;button type=&quot;submit&quot; name=&quot;op&quot; value=&quot;Link 2&quot;&gt;Link 2&lt;/button&gt;&lt;/li&gt;
            &lt;!-- ... --&gt;
          &lt;/ul&gt;
        &lt;/div&gt;
        &lt;!-- The actual form --&gt;
        &lt;div class=&quot;span9&quot;&gt;
          &lt;!-- ... --&gt;
        &lt;/div&gt;
      &lt;/div&gt;
    &lt;/form&gt;

Does Bootstrap have any way to make these `&lt;button&gt;`s look like they were actually `&lt;a&gt;`s?

How to make a &lt;button&gt; in Bootstrap look like a normal link in nav-tabs?

This should be straightforward, but the closest thing I&#39;ve found is this post:
https://stackoverflow.com/questions/18265930/pandas-filling-missing-values-within-a-group, and I still can&#39;t solve my problem....

Suppose I have the following dataframe

    df = pd.DataFrame({&#39;value&#39;: [1, np.nan, np.nan, 2, 3, 1, 3, np.nan, 3], &#39;name&#39;: [&#39;A&#39;,&#39;A&#39;, &#39;B&#39;,&#39;B&#39;,&#39;B&#39;,&#39;B&#39;, &#39;C&#39;,&#39;C&#39;,&#39;C&#39;]})

      name  value
    0    A      1
    1    A    NaN
    2    B    NaN
    3    B      2
    4    B      3
    5    B      1
    6    C      3
    7    C    NaN
    8    C      3

and I&#39;d like to fill in &quot;NaN&quot; with mean value in each &quot;name&quot; group, i.e.

          name  value
    0    A      1
    1    A      1
    2    B      2
    3    B      2
    4    B      3
    5    B      1
    6    C      3
    7    C      3
    8    C      3

I&#39;m not sure where to go after:

    grouped = df.groupby(&#39;name&#39;).mean()

Thanks a bunch.

Pandas: filling missing values by mean in each group

A dot `.` in a regular expression matches any single character. In order for regex to match a dot, the dot has to be escaped: `\.`

[It has been pointed out to me](https://stackoverflow.com/questions/19967972/using-multiple-delimiters-for-split-in-java/19967996#comment29723775_19967996) that inside square brackets `[]` a dot does not have to be escaped. For example, the expression: 
`[.]{3}` would match `...` string.

Doesn&#39;t it, really? And if so, is it true for all regex standards?

Does a dot have to be escaped in a character class (square brackets) of a regular expression?

A dot <code>.</code> in a regular expression matches any single character. In order for regex to match a dot, the dot has to be escaped: <code>\.</code>
<a href="https://stackoverflow.com/questions/19967972/using-multiple-delimiters-for-split-in-java/19967996#comment29723775_19967996" target="_blank" rel="noopener noreferrer">It has been pointed out to me</a> that inside square brackets <code>[]</code> a dot does not have to be escaped. For example, the expression:
<code>[.]{3}</code> would match <code>...</code> string.
Doesn't it, really? And if so, is it true for all regex standards?

I&#39;m making a validator based on [UUID](http://en.wikipedia.org/wiki/Universally_unique_identifier) generated by client browser, I use this to identify a certain type data that the user sends; and would like to validate that the `UUID` that client sends it is in fact a valid [Version 4][1] `UUID`.

I found this https://stackoverflow.com/questions/6223185/php-preg-match-uuid-v4, it&#39;s close  but not exactly what I&#39;m looking for. I wish to know if exists something similar to `is_empty()` or `strtodate()` Where if string is not valid Sends `FALSE`.

I could do based on the regular expression but I would like something more native to test it.

Any ideas?

**11/23/2019 EDIT:** About the duplicate tag, while the moderator is technicallly correct, this question was formulated with the goal of fibd something else to regex if existed, and in second place this question has become a reference to Pythoners and PHPers and has a different answers/approach to solve the problem and their answers are better explained in general. This is why I consider this question should be perserved

  [1]: http://en.wikipedia.org/wiki/Universally_unique_identifier#Version_4_.28random.29

How to determine if a string is a valid v4 UUID?

Is there an alternative to using a regular expression to detect if a string contains uppercase characters? Currently I&#39;m using the following regular expression:

    Regex.IsMatch(fullUri, &quot;[A-Z]&quot;) 

It works fine but I often hear the old adage &quot;If you&#39;re using regular expressions you now have two problems&quot;.


Detect if a string contains uppercase characters

While writing [this answer][1], I had to match exclusively on linebreaks instead of using the `s`-flag (`dotall` - dot matches linebreaks).  

The sites usually used to test regular expressions behave differently when trying to match on `\n` or `\r\n`.

I noticed

 - Regex101 matches linebreaks only on `\n`   
([example][2] - delete `\r` and it matches)  


 - RegExr matches linebreaks _neither_ on `\n` _nor_ on `\r\n`  
and I can&#39;t find something to make it match a linebreak, except for the `m`-flag and `\s`   
([example][3])

 - Debuggex behaves even more different:  
in [this example][4] it matches only on `\r\n`, while  
[here][5] it only matches on `\n`, with the same flags and engine specified

I&#39;m fully aware of the `m`-flag (multiline - makes `^` match the start and `$` the end of a line), but sometimes this is not an option. Same with `\s`, as it matches tabs and spaces, too.    

My thought to use the unicode newline character ([`\u0085`][6]) wasn&#39;t successful, so:

 1. Is there a failsafe way to integrate the match on a linebreak (preferably regardless of the language used) into a regular expression?
 2. Why do the above mentioned sites behave differently (especially Debuggex, matching once only on `\n` and once only on `\r\n`)?


  [1]: https://stackoverflow.com/a/20043441/1807643
  [2]: http://regex101.com/r/yN9wD4
  [3]: http://regexr.com?378ck
  [4]: https://www.debuggex.com/r/uITaeQfcRwdQP_o-
  [5]: https://www.debuggex.com/r/LLeVEF4AhWMUbJ0V
  [6]: http://www.fileformat.info/info/unicode/char/0085/index.htm

Match linebreaks - \n or \r\n?

I can use the SQL `Like` Operator using `pymongo`,

    db.test.find({&#39;c&#39;:{&#39;$regex&#39;:&#39;ttt&#39;}})

But how can I use `Not Like` Operator?

I tried

    db.test.find({&#39;c&#39;:{&#39;$not&#39;:{&#39;$regex&#39;:&#39;ttt&#39;}})

but got error:

&gt; OperationFailure: $not cannot have a regex

How can I use &#39;Not Like&#39; operator in MongoDB

I&#39;m trying to find all values with following pattern :

    value=&quot;4&quot;
    value=&quot;403&quot;
    value=&quot;200&quot;
    value=&quot;201&quot;
    value=&quot;116&quot;
    value=&quot;15&quot;

and replace it with value inside scopes.

I&#39;m using the following regex to find the pattern :

    .*&quot;\d+&quot;

How can I do a replacement?


How to replace all strings to numbers contained in each string in Notepad++?

I&#39;ve just lost three days of my life tracking down a very strange bug where unordered_map::insert() destroys the variable you insert. This highly non-obvious behaviour occurs in very recent compilers only: I found that clang 3.2-3.4 and GCC 4.8 are the **only** compilers to demonstrate this &quot;feature&quot;.

Here&#39;s some reduced code from my main code base which demonstrates the issue:

&lt;!-- language: c++ --&gt;

    #include &lt;memory&gt;
    #include &lt;unordered_map&gt;
    #include &lt;iostream&gt;
    
    int main(void)
    {
      std::unordered_map&lt;int, std::shared_ptr&lt;int&gt;&gt; map;
      auto a(std::make_pair(5, std::make_shared&lt;int&gt;(5)));
      std::cout &lt;&lt; &quot;a.second is &quot; &lt;&lt; a.second.get() &lt;&lt; std::endl;
      map.insert(a); // Note we are NOT doing insert(std::move(a))
      std::cout &lt;&lt; &quot;a.second is now &quot; &lt;&lt; a.second.get() &lt;&lt; std::endl;
      return 0;
    }

I, like probably most C++ programmers, would expect output to look something like this:

    a.second is 0x8c14048
    a.second is now 0x8c14048

But with clang 3.2-3.4 and GCC 4.8 I get this instead:

    a.second is 0xe03088
    a.second is now 0

Which might make no sense, until you examine closely the docs for unordered_map::insert() at http://www.cplusplus.com/reference/unordered_map/unordered_map/insert/ where overload no 2 is:

&lt;!-- language: c++ --&gt;

    template &lt;class P&gt; pair&lt;iterator,bool&gt; insert ( P&amp;&amp; val );

Which is a greedy universal reference move overload, consuming anything not matching any of the other overloads, and *move constructing* it into a value_type. So why did our code above choose this overload, and not the unordered_map::value_type overload as probably most would expect?

The answer stares you in the face: unordered_map::value_type is a pair&lt;*const* int, std::shared_ptr&lt;int&gt;&gt; and the compiler would correctly think that a pair&lt;*int*, std::shared_ptr&lt;int&gt;&gt; isn&#39;t convertible. Therefore the compiler chooses the move universal reference overload, and that destroys the original, **despite** the programmer not using std::move() which is the typical convention for indicating you are okay with a variable getting destroyed. Therefore the insert destroying behaviour is in fact *correct* as per the C++11 standard, and older compilers were *incorrect*.

You can probably see now why I took three days to diagnose this bug. It was not at all obvious in a large code base where the type being inserted into unordered_map was a typedef defined far away in source code terms, and it never occurred to anyone to check if the typedef was identical to value_type.

So my questions to Stack Overflow:

1. Why do older compilers not destroy variables inserted like newer compilers? I mean, even GCC 4.7 doesn&#39;t do this, and it&#39;s pretty standards conforming.

2. Is this problem widely known, because surely upgrading compilers will cause code which used to work to suddenly stop working?

3. Did the C++ standards committee intend this behaviour?

4. How would you suggest that unordered_map::insert() be modified to give better behaviour? I ask this because if there is support here, I intend to submit this behaviour as a N note to WG21 and ask them to implement a better behaviour.

Is it intended by the C++ standards committee that in C++11 unordered_map destroys what it inserts?

Is this valid json?

&lt;!-- language: json --&gt;

    {
        &quot;a&quot; : &quot;x&quot;,
        &quot;a&quot; : &quot;y&quot;
    }

http://jsonlint.com/ says yes.

http://www.json.org/ doesn&#39;t say anything about it being forbidden.

But obviously it doesn&#39;t make much sense, does it?
Most implementations probably use a hashtable so it is being overriden anyways.

Does JSON syntax allow duplicate keys in an object?

Suppose `A`, `B`, `a`, and `b` are all variables, and the addresses of `A`, `B`, `a`, and `b` are all different. Then, for the following code:

    A = a;
    B = b;

Do the C and C++ standard explicitly require `A=a` be strictly executed before `B=b`? Given that the addresses of `A`, `B`, `a`, and `b` are all different, are compilers allowed to swap the execution sequence of two statements for some purpose such as optimization?

If the answer to my question is different in C and C++, I would like to know both.

Edit: The background of the question is the following. In board game AI design, for optimization people use [lock-less shared-hash table](https://chessprogramming.wikispaces.com/Shared+Hash+Table), whose correctness strongly depends on the execution order if we do not add `volatile` restriction.

For { A=a; B=b; }, will &quot;A=a&quot; be strictly executed before &quot;B=b&quot;?

I&#39;m trying to compute the length of a string literal at compile time. To do so I&#39;m using following code:

    #include &lt;cstdio&gt;
    
    int constexpr length(const char* str)
    {
        return *str ? 1 + length(str + 1) : 0;
    }
    
    int main()
    {
        printf(&quot;%d %d&quot;, length(&quot;abcd&quot;), length(&quot;abcdefgh&quot;));
    }

Everything works as expected, the program prints 4 and 8. The assembly code generated by clang shows that the results are computed at compile time:

    0x100000f5e:  leaq   0x35(%rip), %rdi          ; &quot;%d %d&quot;
    0x100000f65:  movl   $0x4, %esi
    0x100000f6a:  movl   $0x8, %edx
    0x100000f6f:  xorl   %eax, %eax
    0x100000f71:  callq  0x100000f7a               ; symbol stub for: printf

**My question: is it guaranteed by the standard that `length` function will be evaluated compile time?** 

If this is true the door for compile time string literals computations just opened for me... for example I can compute hashes at compile time and many more... 

Computing length of a C string at compile time. Is this really a constexpr?

I have a question about PHP syntax.

When defining functions and variables, which convention should I use?

I know they do the same thing in practice but I would like to know which method conforms to best practice standards.

**Variables**

    public $varname = array();

or

    public $varname = [];

**Methods**

    public function foo($bar = array()){}

or

    public function foo($bar = []){}



PHP array vs [ ] in method and variable declaration

Coderbyte is an online coding challenge site (I found it just 2 minutes ago).

[The first C++ challenge][1] you are greeted with has a C++ skeleton you need to modify:

&gt;     #include &lt;iostream&gt;
&gt;     #include &lt;string&gt;
&gt;     using namespace std;
&gt;
&gt;     int FirstFactorial(int num) {
&gt;
&gt;       // Code goes here
&gt;       return num;
&gt;
&gt;     }
&gt;
&gt;     int main() {
&gt;
&gt;       // Keep this function call here
&gt;       cout &lt;&lt; FirstFactorial(gets(stdin));
&gt;       return 0;
&gt;
&gt;     }

If you are little familiar with C++ the first thing&lt;sup&gt;*&lt;/sup&gt; that pops in your eyes is:

```
int FirstFactorial(int num);
cout &lt;&lt; FirstFactorial(gets(stdin));
```

So, ok, the code calls `gets` which is deprecated since C++11 and removed since C++14 which is bad in itself. 

But then I realize: `gets` is of type `char*(char*)`. So it shouldn&#39;t accept a `FILE*` parameter and the result shouldn&#39;t be usable in the place of an `int` parameter, but ... not only it compiles without any warnings or errors, but it runs and actually passes the correct input value to `FirstFactorial`.

Outside of this particular site, the code doesn&#39;t compile (as expected), so what is going on here?

---

&lt;sup&gt;*Actually the first one is `using namespace std` but that is irrelevant to my issue here.&lt;/sup&gt;

  [1]: https://www.coderbyte.com/editor/guest:First%20Factorial:Cpp



What is going on with &#39;gets(stdin)&#39; on the site coderbyte?

what is the difference between curly brace and square bracket in python?

    A ={1,2}
    B =[1,2]

when I print `A` and `B` on my terminal, they made no difference. Is it real?

And sometimes, I noticed some code use `{}` and `[]` to initialize different variables. 

E.g. `A=[]`, `B={}`

Is there any difference there?



What is the difference between curly brace and square bracket in Python?

    func main() {
    	var data = map[string]string{}
    	data[&quot;a&quot;] = &quot;x&quot;
    	data[&quot;b&quot;] = &quot;x&quot;
    	data[&quot;c&quot;] = &quot;x&quot;
    	fmt.Println(data)
    }

It runs.

    func main() {
    	var data = map[string][]string{}
    	data[&quot;a&quot;] = append(data[&quot;a&quot;], &quot;x&quot;)
    	data[&quot;b&quot;] = append(data[&quot;b&quot;], &quot;x&quot;)
    	data[&quot;c&quot;] = append(data[&quot;c&quot;], &quot;x&quot;)
    	fmt.Println(data)
    }

It also runs.

    func main() {
    	var w = map[string]string{}
    	var data = map[string]map[string]string{}
    	w[&quot;w&quot;] = &quot;x&quot;
    	data[&quot;a&quot;] = w
    	data[&quot;b&quot;] = w
    	data[&quot;c&quot;] = w
    	fmt.Println(data)
    }

It runs again!

    func main() {
    	var data = map[string]map[string]string{}
    	data[&quot;a&quot;][&quot;w&quot;] = &quot;x&quot;
    	data[&quot;b&quot;][&quot;w&quot;] = &quot;x&quot;
    	data[&quot;c&quot;][&quot;w&quot;] = &quot;x&quot;
    	fmt.Println(data)
    }

But it fails!?

Is there a problem with nested maps in Go? Or is there no multiple bracket support for nested maps?

Content Type	Original Author	Original Content on Stackoverflow
Question	Dariusz	View Question on Stackoverflow
Solution 1 - Regex	lilactiger89	View Answer on Stackoverflow

Does a dot have to be escaped in a character class (square brackets) of a regular expression?

Regex Problem Overview

Regex Solutions

Solution 1 - Regex

Pandas: filling missing values by mean in each group

How to make a <button> in Bootstrap look like a normal link in nav-tabs?

Attributions