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Alphabets:-
An alphabet is a set of finite , non-empty set of symbol we use the symbol for an alphabet.
eg:

={0,1} is the binary alphabet.

String:-
A string (or sometimes word) is a finite sequence of symbols choose from some alphabets
eg:-

10010 is a string from the binary alphabet ={0,1}

Empty String:-
Empty string is the string with zero occurrences of the symbols denoted by   .Empty string is also called as "Null String".
Length of String:-
The no. of symbols in the string notation for length of string w is |w| -----(carnality)
|a11|=3
||=0
The number of symbol in the sequence of given string is called "length of string".
eg:-
|abb|=3,for the string abb over ∑=(a,b)

Substring of a String:-Any sequence of zero or more consecutive symbols of  the give string over an alphabet is called a "substring of string".
Example:-For string abb over ∑={a,b},the possible substring are

£,a,b,ab,bb and abb.

Prefix of a String:-Any substring with the sequence of beginning symbols of a given string is called a "Prefix".
Example:- For string "abb" ,the possible prefixes of abb are;

£,a,ab,abb.

Suffix of a String:-Any substring with the sequence of trailing(ending) symbols of a given string is called a "Suffix".
Example:- For string "abb" ,the possible suffixed are;
£,b,bb,abb.

Language:- A set of string over the given alphabet ∑ is called a "language" (or collection of string).
Example:-Let ∑={a,b}.Then Language L-{ab,aab}.