What is the logic behind google spellcheck

When I wanted to search a word or some thing in google; If there is some spelling mistake in that word or sentence, google can get back me with correct spell or corrected sentence. Can anyone explain me how exactly this is being done. I will happy if anyone can explain in terms of programming than in terms of开发者_Go百科 database and all those stuff. Thank you.

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Combination of string comparison (with dictionary), stemming and popularity match word base on its large user statistic data.

EDIT: there's a wikipedia page that may helps you understand how computer spell check works.