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In computing and telecommunications, a control character or non-printing character (NPC) is a code point in a character set that does not represent a written character or symbol. They are used as in-band signaling to cause effects other than the addition of a symbol to the text.
In SGML, HTML and XML documents, the logical constructs known as character data and attribute values consist of sequences of characters, in which each character can manifest directly (representing itself), or can be represented by a series of characters called a character reference, of which there are two types: a numeric character reference and a character entity reference.
Restrictions may also apply for other reasons. For example, in HTML 4, , which is a reference to a non-printing "form feed" control character, is allowed because a form feed character is allowed. But in XML, the form feed character cannot be used, not even by reference.
Many Unicode characters are used to control the interpretation or display of text, but these characters themselves have no visual or spatial representation. For example, the null character ( U+0000 NULL ) is used in C-programming application environments to indicate the end of a string of characters.
1 Control-C has typically been used as a "break" or "interrupt" key. 2 Control-D has been used to signal "end of file" for text typed in at the terminal on Unix / Linux systems. Windows, DOS, and older minicomputers used Control-Z for this purpose. 3 Control-G is an artifact of the days when teletypes were in use. Important messages could be ...
A numeric character reference in HTML refers to a character by its Universal Character Set/Unicode code point, and uses the format &#nnnn; or &#xhhhh; where nnnn is the code point in decimal form, and hhhh is the code point in hexadecimal form. The x must be lowercase in XML documents.
The LRM control character causes the punctuation to be adjacent to only left-to-right text – the "C" and the LRM – and position as if it were in left-to-right text, i.e., to the right of the preceding text. Some software requires using the HTML code ‎ or ‎ instead of the invisible Unicode control character itself.
For example, it enables a Hebrew quote in an English text. The Bidi_Character_Type marks a character's behaviour in directional writing. To override a direction, Unicode has defined special formatting control characters (Bidi-Controls). These characters can enforce a direction, and by definition only affect bi-directional writing.