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A **substring** is a contiguous sequence of characters within a string. For instance, "*the best of*" is a substring of "*It was the best of times*". This is not to be confused with subsequence, which is a generalization of substring. For example, "*Itwastimes*" is a subsequence of "*It was the best of times*", but not a substring.

**Prefix** and **suffix** are special cases of substring. A prefix of a string is a substring of that occurs at the *beginning* of . A suffix of a string is a substring that occurs at the *end* of .

The list of all substrings of the string "*apple*" would be "*apple*", "*appl*", "*pple*", "*app*", "*ppl*", "*ple*", "*ap*", "*pp*", "*pl*", "*le*", "*a*", "*p*", "*l*", "*e*", "".

A substring (or factor) of a string is a string , where and . A substring of a string is a prefix of a suffix of the string, and equivalently a suffix of a prefix. If is a substring of , it is also a subsequence, which is a more general concept. Given a pattern , you can find its occurrences in a string with a string searching algorithm. Finding the longest string which is equal to a substring of two or more strings is known as the longest common substring problem.

Example: The string `ana`

is equal to substrings (and subsequences) of `banana`

at two different offsets:

banana ||||| ana|| ||| ana

In the mathematical literature, substrings are also called **subwords** (in America) or **factors** (in Europe).

Not including the empty substring, the number of substrings of a string of length where symbols only occur once, is the number of ways to choose two distinct places between symbols to start/end the substring. Including the very beginning and very end of the string, there are such places. So there are non-empty substrings.

A prefix of a string is a string , where . A *proper prefix* of a string is not equal to the string itself ();^{[1]} some sources^{[2]} in addition restrict a proper prefix to be non-empty (). A prefix can be seen as a special case of a substring.

Example: The string `ban`

is equal to a prefix (and substring and subsequence) of the string `banana`

:

banana ||| ban

The square subset symbol is sometimes used to indicate a prefix, so that denotes that is a prefix of . This defines a binary relation on strings, called the prefix relation, which is a particular kind of prefix order.

In formal language theory, the term *prefix of a string* is also commonly understood to be the set of all prefixes of a string, with respect to that language.

A suffix of a string is any substring of the string which includes its last letter, including itself. A *proper suffix* of a string is not equal to the string itself. A more restricted interpretation is that it is also not empty^{[1]}. A suffix can be seen as a special case of a substring.

Example: The string `nana`

is equal to a suffix (and substring and subsequence) of the string `banana`

:

banana |||| nana

A suffix tree for a string is a trie data structure that represents all of its suffixes. Suffix trees have large numbers of applications in string algorithms. The suffix array is a simplified version of this data structure that lists the start positions of the suffixes in alphabetically sorted order; it has many of the same applications.

A border is suffix and prefix of the same string, e.g. "bab" is a border of "babab" (and also of "babooneatingakebab").

A **superstring** of a finite set of strings is a single string that contains every string in as a substring. For example, is a superstring of , and is a shorter one. Generally, one is interested in finding superstrings whose length is as small as possible;^{[clarification needed]} a concatenation of all strings of in any order gives a trivial superstring of .

**^**Kelley, Dean (1995).*Automata and Formal Languages: An Introduction*. London: Prentice-Hall International. ISBN 0-13-497777-7.**^**Gusfield, Dan (1999) [1997].*Algorithms on Strings, Trees and Sequences: Computer Science and Computational Biology*. USA: Cambridge University Press. ISBN 0-521-58519-8.

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