This shouldn&#39;t match anything:

    re.compile(&#39;$^&#39;)

So if you replace regexp1, regexp2 and regexp3 with &#39;$^&#39; it will be impossible to find a match. Unless you are using the multi line mode.


----------


After some tests I found a better solution

    re.compile(&#39;a^&#39;)

It is impossible to match and will fail earlier than the previous solution. You can replace a with any other character and it will always be impossible to match

`(?!)` should always fail to match.  It is the zero-width negative look-ahead.  If what is in the parentheses matches then the whole match fails.  Given that it has nothing in it, it will fail the match for anything (including nothing).

To match an empty string - even in multiline mode - you can use `\A\Z`, so:

    re.compile(&#39;\A\Z|\A\Z*|\A\Z+&#39;)


The difference is that `\A` and `\Z` are start and end of *string*, whilst `^` and `$` these can match start/end of *lines*, so `$^|$^*|$^+` could potentially match a string containing newlines (if the flag is enabled).


And to fail to match anything (even an empty string), simply attempt to find content before the start of the string, e.g:

    re.compile(&#39;.\A|.\A*|.\A+&#39;)

Since no characters can come before \A (by definition), this will always fail to match.

You could use  
`\z..`  
This is the absolute end of string, followed by two of anything

If `+` or `*` is tacked on the end this still works refusing to match anything  

Or, use some list comprehension to remove the useless regexp entries and join to put them all together. Something like:

    re.compile(&#39;|&#39;.join([x for x in [regexp1, regexp2, ...] if x != None]))

Be sure to add some comments next to that line of code though :-)

In numpy one can use the &#39;newaxis&#39; object in the slicing syntax to create an axis of length one, e.g.:

    import numpy as np
    print np.zeros((3,5))[:,np.newaxis,:].shape
    # shape will be (3,1,5)

The [documentation states][1] that one can also use `None` instead of `newaxis`, the effect is exactly the same.

Is there any reason to choose one over the other? Is there any general preference or style guide? My impression is that `newaxis` is more popular, probably because it is more explicit. So is there any reason why `None` is allowed?


  [1]: http://docs.scipy.org/doc/numpy/reference/arrays.indexing.html#numpy.newaxis

Numpy: Should I use newaxis or None?

I am trying to read a CSV file with accented characters with Python (only French and/or Spanish characters). Based on the Python 2.5 documentation for the csvreader (http://docs.python.org/library/csv.html), I came up with the following code to read the CSV file since the csvreader supports only ASCII.

    def unicode_csv_reader(unicode_csv_data, dialect=csv.excel, **kwargs):
        # csv.py doesn&#39;t do Unicode; encode temporarily as UTF-8:
        csv_reader = csv.reader(utf_8_encoder(unicode_csv_data),
                                dialect=dialect, **kwargs)
        for row in csv_reader:
            # decode UTF-8 back to Unicode, cell by cell:
            yield [unicode(cell, &#39;utf-8&#39;) for cell in row]
    
    def utf_8_encoder(unicode_csv_data):
        for line in unicode_csv_data:
            yield line.encode(&#39;utf-8&#39;)
    
    filename = &#39;output.csv&#39;
    reader = unicode_csv_reader(open(filename))
    try:
    	products = []
    	for field1, field2, field3 in reader:
            ...

Below is an extract of the CSV file I am trying to read:

    0665000FS10120684,SD1200IS,Appareil photo num&#233;rique PowerShot de 10 Mpx de Canon avec tr&#233;pied (SD1200IS) - Bleu
    0665000FS10120689,SD1200IS,Appareil photo num&#233;rique PowerShot de 10 Mpx de Canon avec tr&#233;pied (SD1200IS) - Gris
    0665000FS10120687,SD1200IS,Appareil photo num&#233;rique PowerShot de 10 Mpx de Canon avec tr&#233;pied (SD1200IS) - Vert
    ...

Even though I try to encode/decode to UTF-8, I am still getting the following exception:

    Traceback (most recent call last):
      File &quot;.\Test.py&quot;, line 53, in &lt;module&gt;
        for field1, field2, field3 in reader:
      File &quot;.\Test.py&quot;, line 40, in unicode_csv_reader
        for row in csv_reader:
      File &quot;.\Test.py&quot;, line 46, in utf_8_encoder
        yield line.encode(&#39;utf-8&#39;, &#39;ignore&#39;)
    UnicodeDecodeError: &#39;ascii&#39; codec can&#39;t decode byte 0xc3 in position 68: ordinal not in range(128)

How do I fix this?

Reading a UTF8 CSV file with Python

I have a python template engine that heavily uses regexp. It uses concatenation like:

    re.compile( regexp1 + &quot;|&quot; + regexp2 + &quot;*|&quot; + regexp3 + &quot;+&quot; )

I can modify the individual substrings (regexp1, regexp2 etc).

Is there any small and light expression that matches nothing, which I can use inside a template where I don&#39;t want any matches? Unfortunately, sometimes &#39;+&#39; or &#39;*&#39; is appended to the regexp atom so I can&#39;t use an empty string - that will raise a &quot;nothing to repeat&quot; error.

Regular expression syntax for &quot;match nothing&quot;?

I have a python template engine that heavily uses regexp. It uses concatenation like:
<pre><code class="hljs language-python">re.compile( regexp1 + "|" + regexp2 + "*|" + regexp3 + "+" )
</code></pre>
I can modify the individual substrings (regexp1, regexp2 etc).
Is there any small and light expression that matches nothing, which I can use inside a template where I don't want any matches? Unfortunately, sometimes '+' or '*' is appended to the regexp atom so I can't use an empty string - that will raise a "nothing to repeat" error.

Using Python, I&#39;d like to compare every possible pair in a list.

Suppose I have

    my_list = [1,2,3,4]

I&#39;d like to do an operation (let&#39;s call it foo) on every combination of 2 elements from the list.

The final result should be the same as

    foo(1,1)
    foo(1,2)
    ...
    foo(4,3)
    foo(4,4)


My first thought was to iterate twice through the list manually, but that doesn&#39;t seem very pythonic.

Operation on every pair of element in a list

1. Is there a performance or code maintenance issue with using `assert` as part of the standard code instead of using it just for debugging purposes?

 Is

        assert x &gt;= 0, &#39;x is less than zero&#39;

 better or worse than 

        if x &lt; 0:
            raise Exception, &#39;x is less than zero&#39;

2. Also, is there any way to set a business rule like `if x &lt; 0 raise error` that is always checked without the `try/except/finally` so, if at anytime throughout the code `x` is less than 0 an error is raised, like if you set `assert x &lt; 0` at the start of a function, anywhere within the function where `x` becomes less then 0 an exception is raised?


Best practice for using assert?

`float(&#39;nan&#39;)` represents NaN (not a number). But how do I check for it?

How can I check for NaN values?

Python&#39;s list type has an index() method that takes one parameter and returns the index of the first item in the list matching the parameter.  For instance:

    &gt;&gt;&gt; some_list = [&quot;apple&quot;, &quot;pear&quot;, &quot;banana&quot;, &quot;grape&quot;]
    &gt;&gt;&gt; some_list.index(&quot;pear&quot;)
    1
    &gt;&gt;&gt; some_list.index(&quot;grape&quot;)
    3

Is there a graceful (idiomatic) way to extend this to lists of complex objects, like tuples?  Ideally, I&#39;d like to be able to do something like this:

    &gt;&gt;&gt; tuple_list = [(&quot;pineapple&quot;, 5), (&quot;cherry&quot;, 7), (&quot;kumquat&quot;, 3), (&quot;plum&quot;, 11)]
    &gt;&gt;&gt; some_list.getIndexOfTuple(1, 7)
    1
    &gt;&gt;&gt; some_list.getIndexOfTuple(0, &quot;kumquat&quot;)
    2


getIndexOfTuple() is just a hypothetical method that accepts a sub-index and a value, and then returns the index of the list item with the given value at that sub-index.  I hope

Is there some way to achieve that general result, using list comprehensions or lambas or something &quot;in-line&quot; like that?  I think I could write my own class and method, but I don&#39;t want to reinvent the wheel if Python already has a way to do it.


Using Python&#39;s list index() method on a list of tuples or objects?

I am saving user-submitted HTML (in a database). I must prevent JavaScript injection attacks.  The most pernicious I have seen is JavaScript in a `style=&quot;expression(...)&quot;`.

In addition to this, a fair amount of valid user content will include special characters and XML constructs, so I&#39;d like to avoid a white-list approach if possible. (Listing every allowable HTML element and attribute).

Examples of JavaScript attack strings:

1.
```
&quot;Hello, I have a
&amp;lt;script&amp;gt;alert(&quot;bad!&quot;)&amp;lt;/script&amp;gt;
problem with the &amp;lt;dog&amp;gt;
element...&quot;
```

2.
```
&quot;Hi, this &amp;lt;b
style=&quot;width:expression(alert(&#39;bad!&#39;))&quot;&amp;gt;dog&amp;lt;/b&amp;gt;
is black.&quot;
```

Is there a way to prevent such JavaScript, and leave the rest intact?

The only solution I have so far is to use a regular expression to remove certain patterns. It solves case 1, but not case 2.

The environment is essentially the Microsoft stack:

- SQL Server 2005 
- C# 3.5  (ASP.NET)
- JavaScript and jQuery.

I would like the chokepoint to be the ASP.NET layer - anyone can craft a bad HTTP request.

### Edit

Thanks for the links, everyone. Assuming that I can define my list (the content will include many mathematical and programming constructs, so a whitelist is going to be very annoying), I still have a question:

What kind of parser will allow me to just remove the &quot;bad&quot; parts?  The bad part could be an entire element, but then what about those scripts that reside in the attributes? I can&#39;t remove `&lt; a hrefs &gt;` willy-nilly.

How to prevent Javascript injection attacks within user-generated HTML

In R, is it possible to extract group capture from a regular expression match?  As far as I can tell, none of `grep`, `grepl`, `regexpr`, `gregexpr`, `sub`, or `gsub` return the group captures.

I need to extract key-value pairs from strings that are encoded thus:

    $(.*?) :: (0\.[0-9]+)$

I can always just do multiple full-match greps, or do some outside (non-R) processing, but I was hoping I can do it all within R.  Is there&#39;s a function or a package that provides such a function to do this?

Regex group capture in R with multiple capture-groups

I have the following template String: `&quot;Hello [Name] Please find attached [Invoice Number] which is due on [Due Date]&quot;`.

I also have String variables for name, invoice number and due date - what&#39;s the best way to replace the tokens in the template with the variables?  

(Note that if a variable happens to contain a token it should NOT be replaced).

----------

**EDIT**

With thanks to @laginimaineb and @alan-moore, **here&#39;s my solution:**

    public static String replaceTokens(String text, 
                                       Map&lt;String, String&gt; replacements) {
        Pattern pattern = Pattern.compile(&quot;\\[(.+?)\\]&quot;);
        Matcher matcher = pattern.matcher(text);
        StringBuffer buffer = new StringBuffer();

        while (matcher.find()) {
            String replacement = replacements.get(matcher.group(1));
            if (replacement != null) {
                // matcher.appendReplacement(buffer, replacement);
                // see comment 
                matcher.appendReplacement(buffer, &quot;&quot;);
                buffer.append(replacement);
            }
        }
        matcher.appendTail(buffer);
        return buffer.toString();
    }

How to replace a set of tokens in a Java String?

I&#39;m attempting something which I feel should be fairly obvious to me but it&#39;s not.  I&#39;m trying to match a string which does NOT contain a specific sequence of characters.  I&#39;ve tried using `[^ab]`, `[^(ab)]`, etc. to match strings containing no &#39;a&#39;s or &#39;b&#39;s, or only &#39;a&#39;s or only &#39;b&#39;s or &#39;ba&#39; but not match on &#39;ab&#39;.  The examples I gave won&#39;t match &#39;ab&#39; it&#39;s true but they also won&#39;t match &#39;a&#39; alone and I need them to.  Is there some simple way to do this?

Regular Expressions and negating a whole character group

I have a table with ~500k rows; varchar(255) UTF8 column `filename` contains a file name;

I&#39;m trying to strip out various strange characters out of the filename - thought I&#39;d use a character class: `[^a-zA-Z0-9()_ .\-]`

Now, **is there a function in MySQL that lets you replace through a regular expression**? I&#39;m looking for a similar functionality to REPLACE() function - simplified example follows:
    
    SELECT REPLACE(&#39;stackowerflow&#39;, &#39;ower&#39;, &#39;over&#39;);
    
    Output: &quot;stackoverflow&quot;

    /* does something like this exist? */
    SELECT X_REG_REPLACE(&#39;Stackoverflow&#39;,&#39;/[A-Zf]/&#39;,&#39;-&#39;); 

    Output: &quot;-tackover-low&quot;

I know about [REGEXP/RLIKE][1], but those only check *if* there is a match, not *what* the match is.

(I *could* do a &quot;`SELECT pkey_id,filename FROM foo WHERE filename RLIKE &#39;[^a-zA-Z0-9()_ .\-]&#39;`&quot; from a PHP script, do a `preg_replace` and then &quot;`UPDATE foo ... WHERE pkey_id=...`&quot;, but that looks like a last-resort slow &amp; ugly hack)


  [1]: https://stackoverflow.com/a/6943142/19746

Content Type	Original Author	Original Content on Stackoverflow
Question	grigoryvp	View Question on Stackoverflow
Solution 1 - Python	Nadia Alramli	View Answer on Stackoverflow
Solution 2 - Python	Chas. Owens	View Answer on Stackoverflow
Solution 3 - Python	Peter Boughton	View Answer on Stackoverflow
Solution 4 - Python	Steef	View Answer on Stackoverflow
Solution 5 - Python	ShuggyCoUk	View Answer on Stackoverflow
Solution 6 - Python	Mike Miller	View Answer on Stackoverflow

Regular expression syntax for "match nothing"?

Python Problem Overview

Python Solutions

Solution 1 - Python

Solution 2 - Python

Solution 3 - Python

Solution 4 - Python

Solution 5 - Python

Solution 6 - Python

Reading a UTF8 CSV file with Python

Numpy: Should I use newaxis or None?

Attributions