CHARACTER CONSTANT

A character is a letter, numeral or special symbol, which can be handled by the computer 
system.These available symbols define the system's character set.Enclosing a single character 
from the system's character set within single quotation marks forms a character constant. The 
characters used in C language are grouped into three classes .

• Alphabetic characters (a,b,c,....z A,B,C,......Z)
• Numeric characters (0 through 9)
• Special characters (+ - * / % # = ,=.=' " () [] : )

eg:

'1'.'a','+', and '-' are the valid character constants.
 since two single quotes are used to represent the character constant, how do we represent the 
single quote itself as a character constant? It is not possible to represent a single quote character 
enclosed between two single quotes, that is, the representation ''' is invalid.
An escape sequence maybe used to represent a single quote as a  character constant. Character 
combinations consisting of a backslash \ followed by a letter are called escape sequences.

'\'' is a valid single quote character constant .
similarly some non printable character are represented using escape sequence characters.
eg:
'\a'   bell(beep)
'\b'   backspace 
'\f'    forum feed
'\r'    carriage return
'\n'   newline 
'\0'   null character
'\t'    gap

To represent the backslash itself as a character, two backslashes are used('\\')