What is escaping in html. Escaping special characters in regular expressions

Escaping characters- replacing control characters in the text with corresponding text substitutions. One of the types of control sequences.

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Definition

Typically programming languages, text command interfaces, text markup languages ​​(HTML, TeX, wiki markup) deal with structured text in which some characters (and their combinations) are used as managers, including those that control the structure of the text. In a situation where it is necessary to use such a symbol as a “regular language symbol”, use shielding.

Conventionally, shielding can be divided into three types:

  • single character escaping
  • escaping a group of characters using the character sequence “start escaping”, “end escaping”
  • using the command sequence “start escaping” and the “end escaping” character, which is specified before the start of the text being escaped.

Lack of shielding as a cause of vulnerability

Escaping characters attracts Special attention, when structured text is generated automatically. Including arbitrary string data in text requires escaping control characters in it. At the same time, very often real strings do not contain such characters, which allows the programmer to skip this operation completely and get more a simple program, which works correctly with “any reasonable” string data. However, such simplified code has a hidden vulnerability, because a third party (the author of the string data) receives an unauthorized opportunity to influence structure generated text. The vulnerability becomes serious if the generated text is someone else's program. Traditionally, systems using SQL (see SQL-injection) and HTML (see Cross Site Scripting) languages ​​are susceptible to such problems.

Examples

Escaping a Single Character

  • In the C programming language, characters within strings are escaped using the " " character placed before the character being escaped. (In this case, the character "\" can escape itself, that is, the combination "\\" is used to output a backslash), the same character is used to escape characters in command line unix.
  • In the Microsoft Windows command line, escaping some characters is done using the "^" character placed before the character being escaped.

Escaping a group of characters

  • In the Python programming language, escaping a group of characters in a line is carried out by specifying the letter r (from the English raw - unprocessed) before the line, i.e. characters are escaped by sequences r" escape text "
  • In wiki markup, text is escaped using pseudo tags. And. If you need to write down the pseudo tag itself , this is done with wildcard characters ( ).

To understand when and what to avoid without trying, you need to understand exactly the chain of contexts through which the string passes. You will specify the string from the farthest side to its final destination, which is the memory processed by the regular expression parsing code.

Remember how a line in memory is handled: if it could be a simple line inside the code or a line entered into the command line, but could be either an interactive command line, a command line specified in a shell script file, or inside a memory variable mentioned by the code , or a (string) argument upon further evaluation, or a string containing code generated dynamically with any encapsulation...

Each of this context is assigned several symbols with special functionality.

If you want to pass a character literally without using its special function (local to the context), then in that case you have to escape it for the next context... which may need some other escape characters that may need to be additionally escaped in the previous context (Oh). Additionally, there may be things like character encoding (the most insidious is utf-8 because it looks like ASCII for common characters, but can additionally be interpreted even by the terminal depending on its settings, so it may behave differently. than the HTML/XML encoding attribute, this is necessary to properly understand the process.

For example, a regular expression on the command line starting with perl -npe should be passed to a set of exec system calls connecting as a pipe that processes the file, each of these exec system calls simply has a list of arguments that have been separated by (non-escaped) spaces and perhaps pipes (|) and redirection (>N>N>&M), brackets, interactive expansion * and ? , $(()) ... (these are all special characters used by *sh, which may appear to interfere with the regex character in the following context, but they are evaluated in order: before the command line. The command line is read by the program as bash/sh/csh /tcsh/zsh is essentially inside a double quote or single quote, escaping is easier, but there is no need to quote the string on the command line because basically the space must be prefixed with a backslash and the quote is not necessary, leaving the expansion functionality available for the * and ? characters, but this parses the same context as the quote.Then, when evaluated on the command line, the regular expression produced in memory (not as written on the command line) receives the same treatment as in the source file. For a regular expression in square brackets there is a character set context, a perl regular expression can be enclosed in big set non-alpha numeric characters (e.g. m//or m:/better/for/path:...).

You have more details about characters in another answer that are very specific to the final regex context. As I noted, you mention that you are finding that the regexp is being reset with tries, which is probably because the other context has a different character set that has confused your memory of the tries (often a backslash is the character used in this other context to escape a literal character instead of its function.).

Typically, programming languages, text command interfaces, text markup languages ​​(HTML, TeX, wiki markup) deal with structured text in which some characters (and their combinations) are used as managers, including those that control the structure of the text. In a situation where it is necessary to use such a symbol as a “regular language symbol”, use shielding.

Conventionally, shielding can be divided into three types:

  • single character escaping
  • escaping a group of characters using the character sequence “start escaping”, “end escaping”
  • using the command sequence “start escaping” and the “end escaping” character, which is specified before the start of the text being escaped.

Lack of shielding as a cause of vulnerability

Character escaping is of particular concern when structured text is generated automatically. Including arbitrary string data in text requires escaping control characters in it. At the same time, very often real strings do not contain such characters, which allows the programmer to skip this operation completely and get a simpler program that works correctly with “any reasonable” string data. However, such simplified code has a hidden vulnerability, because a third party (the author of the string data) receives an unauthorized opportunity to influence structure generated text. The vulnerability becomes serious if the generated text is someone else's program. Traditionally, systems using SQL (see SQL injection) and HTML (see Cross Site Scripting) languages ​​are prone to such problems.

Examples

Escaping a Single Character

  • In the C programming language, characters within strings are escaped using the " " character placed before the character being escaped. (In this case, the “\” character can escape itself, that is, the “\\” combination is used to output a backslash), the same character is used to escape characters on the unix command line.
  • In the Microsoft Windows command line, escaping some characters is done using the "^" character placed before the character being escaped.

Escaping a group of characters

  • In the Python programming language, escaping a group of characters in a line is carried out by specifying the letter r (from the English raw - unprocessed) before the line, i.e. characters are escaped by sequences r" escape text "
  • In wiki markup, text is escaped using pseudo tags. And. If you need to write down the pseudo tag itself , this is done with wildcard characters ( ).

Escaping text with a trailing character

When there are a lot of control characters in the text, there will be a lot of escape characters, the text becomes heavy. For such cases, an alternative escaping method is used - with final text. In this case, all control characters will be just characters (they do not carry a control function), and the text ends when the compiler detects a certain sequence - the final text.

3.1 Escaping special characters

Before passing the values ​​of form variables into SQL queries, you need to specially escape some characters in them (in particular, an apostrophe), for example, put a backslash in front of them. The function for inserting is:

mysql_escape_string()

string mysql_escape_string(string $str)

The function is similar to the other addslashes() function, but it adds slashes before a more complete set of special characters. Practice shows that for text data you can use the addslashes() function instead of mysql_escape_string(). This is done in many scripts.

According to the MySQL standard, characters that are written in PHP as follows: "\x00", "\n", "\r", "\\", """, "" and "\x1A" are escaped.

This number includes a character with ASCII code zero, and therefore mysql_escape_string() can be used not only for text, but also for binary data. You can, for example, read a GIF image into a variable (file_get_contents() function), and then insert it into the database, having previously escaped all special characters. When extracted, the image will appear in the same form in which it was originally.

Escaping characters is just a way to write correct SQL expressions, nothing more. Nothing happens to the data, and it is stored in the database without additional slashes - just as it looked initially, even before escaping.

Using mysql_escape_string() the previous query code looks like this:

"DELETE FROM table WHERE name="".mysql_escape_string($name).""");

It's long, clunky and ugly.


3.2 Query templates and placeholders

Let's consider another solution.

Instead of explicitly escaping and inserting variables into the query, special markers (placeholders) are placed in their place, usually looking like ?.

The same values ​​that will be substituted for them are transmitted separately, additional parameters.

Using the hypothetical mysql_qwo function, the code for which will be presented below, the previous query could be rewritten as follows:

mysql_qw("DELETE FROM table WHERE name=?", $name);

The query has become shorter and better protected: now, when writing code, we will not be able to accidentally miss a call to the mysql_escape_string() function and, thus, fall for a hacker’s trick. All transformations occur automatically, inside the function.

The listing for lib_mysql_qw.php contains simplest implementation mysql_qw() functions (qw - from the English query wrapper, “query wrapper”).

There is also a lib/Placeholder.php library that provides much more powerful support for the placeholders language: http://dklab.ru/chicken/30.html.

In most situations, the capabilities provided by the mysql_qw() function are sufficient.

Listing lib_mysql_qw.php

// result-set, mysql_qw ($connection_id, $query, $argl, $arg2 ...).

// result-set mysql_qw($query, $argl, $arg2, ...)

// The function performs a query to MySQL over the connection specified as

// $connection_id (if not specified, then through the last open one).

// $query parameter can contain wildcards?,

// instead of which the corresponding values ​​will be substituted

// arguments $arg1, $arg2, etc. (in order), escaped and

// enclosed in apostrophes.

function mysql_qw()

// Get all the function arguments.

$args = func_get_args();

// If the first parameter is of type "resource", then this is the connection ID.

// Generate a request using a template.

// Call the SQL function.

// string mysql_make_qw($query, $argl, $arg2,...)

// This function generates an SQL query using the $query template,

function mysql_make_qw()

$args = func_get_args();

// After this $args will also be changed.

// Now we escape all the arguments except the first one.

foreach ($args as $i=>$v)

if (!$i) continue; // this is a template

if (is_int($v)) continue; // integers don't need to be escaped

//Just in case, fill the last 20 arguments with invalid ones

// values, so that if the number "?" exceeds the number

// parameters, an SQL query error was thrown (will help with debugging).

for ($i=$c=count($args)-1; $i<$c+20; $i++)

// Form an SQL query.


If you remove the explanatory entries, the size of the lib_mysql_qw.php file will decrease by almost three times:

function mysql_qw()

$args = func_get_args();

if (is_resource($args)) $conn = array_shift($args);

$query = call_user_func_array("mysql_make_qw", $args);

return $conn!==null ? mysql_query($query, $conn): mysql_query($query);

function mysql_make_qw()

$args = func_get_args();

$tmp1 - str_replace("%", "%%", $tmp1);

$tmp1 = str_replace("?", "%s", $tmp1);

foreach ($args as $i=>$v)

if (!$i) continue;

if (is_int($v)) continue;

$args[$i] = """.mysql_escape_string($v).""";

for ($i=$c=count($args)-1; $i<$c+20; $i++)

$args[$i+1] = "UNKNOWN_PLACEHOLDER_$i";

return call_user_func_array("sprintf", $args);


The sprintf() function treats the % character as a control character. To cancel its special action, you need to double it, which is done in the function. Then? is replaced by %s, for sprintf() this means "take another string argument".

To make it easier to test this code, the main function is divided into two, and the code for replacing wildcards in the mysql_make_qw() function is highlighted.

The test_qw.php listing shows an example of what SQL queries will look like after substituting placeholders.

Listing test_qw.php

require_once "lib_mysql_qw.php";

require_once "mysql_connect.php";

// Let's imagine that we are hackers...

$name = "" OR "1";

// Valid request.

echo mysql_make_qw("DELETE FROM people WHERE name=?", $name)."
";

// Invalid request.

echo mysql_make_qw("DELETE FROM people WHERE name=? OR ?, $name)."
";

// This is what query execution looks like.

mysql_qw("DELETE FROM people WHERE name=? OR ?", $name)

or die(mysql_error());

As a result of the script, the following page will be generated:

DELETE FROM people WHERE name="\" OR \"1"

DELETE FROM people WHERE name=" \ " OR \ " 1" OR id=UNKNOWN_PLACEHOLDER_l

Unknown column "UNKNOWN_PLACEHOLDER_1" in "where clause1


Slashes appeared before apostrophes in the data, and placeholder, which “didn’t have enough” function arguments, was replaced with the line UNKNOWN_PLACEHOLDER_l.

Now any attempt to execute such a query is doomed to fail (as indicated by the last diagnostic message generated by the die() call), which is an important aid when debugging scripts.






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From the author: Greetings, friends. In this article we will talk about escaping special characters in regular expressions. By special characters, of course, we mean metacharacters in regular expressions. Shall we begin?

So, as we already know from previous articles, regular expressions contain many different metacharacters, thanks to which all the power of regular expressions is achieved. For example, one of the most commonly used metacharacters is the period. A period in standard pattern mode matches any character except a newline.

This is great, but if we need to find exactly a period in a string, then using a metacharacter will give us a completely different result.

Instead of a couple of dots in a line, we got the entire line. To solve the problem, it is enough to indicate in the regular expression that the period should not be a special character, i.e. so that it coincides only with itself. This is done using another metacharacter, it should be familiar to you, this is a backslash - \.

Actually, this special character is used as an escape character not only in regular expressions, but in other programming languages. So, let's try to put a backslash before the period.

Now everything works as we need. In exactly the same way, we must escape any other metacharacters if necessary if we want them to be treated as regular characters and match only themselves.

Well, that's all for me today. You can learn more about regular expressions in our regular expressions course. Good luck!

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