Redirected from String search algorithms
Let Σ be an alphabet (finite set). Formally, both the pattern and searched text are concatenation of elemements of Σ. The Σ may be usual human alphabet (AZ). Other applications may use binary alphabet (Σ = {0,1}) or DNA alphabet (Σ = {A,C,G,T}) in bioinformatics.

The various algorithms can be classified by the number of patterns each uses.
Preprocessing Time  Matching Time  
Naïve string search algorithm  0 (no preprocessing)  O((nm+1)m) 
RabinKarp algorithm  θ(m)  O((nm+1)m) 
Finite Automata  O(mΣ)  θ(n) 
KnuthMorrisPratt algorithm  θ(m)  θ(n) 
BoyerMoore string search algorithm  
BaezaYates and Gonnet string search algorithm 
Naturally, the patterns can not be enumerated in this case. They are represented usually by a regular grammar or regular expression.
Other classification approaches are possible. One of the most common uses preprocessing as main criteria.
Text not preprocessed  Text preprocessed  
Patterns not preprocessed  Elementary algorithms  Index methods 
Patterns preprocessed  Constructed search engines  Signature methods 
The simplest and least efficient way to see where one string occurs inside another is to check each place it could be, one by one, to see if it's there. So first we see if there's a copy of the needle in the first few characters of the haystack; if not, we look to see if there's a copy of the needle starting at the second character of the haystack; if not, we look starting at the third character, and so forth. In the normal case, we only have to look at one or two characters for each wrong position to see that it's a wrong position, so in the average case, this takes O(n + m) steps, where n is the length of the haystack and m is the length of the needle; but in the worst case, searching for a string like "aaaab" in a string like "aaaaaaaaab", it takes O(nm) steps.
KMP computes a deterministic finite state automaton that recognizes inputs with the string to search for as a suffix, so it doesn't need to back up. BoyerMoore starts searching from the end of the needle, so it can usually jump ahead a whole needlelength at each step. BaezaYates and Gonnet uses bits in a word to keep track of whether the previous N characters were a prefix of the search string, and is therefore adaptable to fuzzy matching etc. These descriptions are insufficient!
Could somebody expand this entry ?
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