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Spell checker
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In computing, a spell checker is an applications program that flags words in a document that may not be spelled correctly. Spell checkers may be stand-alone capable of operating on a block of text, or as part of a larger application, such as a word processor, email client, electronic dictionary, or search engine.
OperationSimple spell checkers operate on individual words by comparing each of them against the contents of a dictionary, possibly performing stemming on the word. If the word is not found it is considered to be a error, and an attempt may be made to suggest a word that was likely to have been intended. One such suggestion algorithm is to list those words in the dictionary having a small Levenshtein distance from the original word.
When a word which is not within the dictionary is encountered most spell checkers provide an option to add that word to a list of known exceptions that should not be flagged.
DesignA spell checker customarily consists of two parts:
- A set of routines for scanning text and extracting words, and
- An algorithm for comparing the extracted words against a known list of correctly spelled words (ie., the dictionary).
The scanning routines sometimes include language-dependent algorithms for handling morphology. Even for a lightly inflected language like English, word extraction routines will need to handle such phenomena as contractions and possessives. It is unclear whether morphological analysis provides a significant benefit.
The word list might contain just a list of words, or it might also contain additional information, such as hyphenation points or lexical and grammatical attributes.
As an adjunct to these two components, the program's user interface will allow users to approve replacements and modify the program's operation.
One exception to the above paradigm are spell checkers which use based solely statistical information, for instance using n-grams. This approach usually requires a lot of effort to obtain sufficient statistical information and may require a lot more runtime storage. These methods are not currently in general use. In some cases spell checkers use a fixed list of misspellings and suggestions for those misspellings; this less flexible approach is often used in paper-based correction methods, such as the see also entries of encyclopedias.
HistoryThe first spell checkers were widely available on mainframe computers in the late 1970s. A group of six linguists from Georgetown University developed the first spell-check system for the IBM corporation. The first spell checkers for personal computers appeared for CP/M and TRS-80 computers in 1980, followed by packages for the IBM PC after it was introduced in 1981. Developers such as Maria Mariani, Soft-Art, Microlytics, Proximity, Circle Noetics, and Reference Software rushed OEM packages or end-user products into the rapidly expanding software market, primarily for the PC but also for Apple Macintosh, VAX, and Unix. On the PCs, these spell checkers were standalone programs, many of which could be run in TSR mode from within word-processing packages on PCs with sufficient memory.
However, the market for standalone packages was short-lived, as by the mid 1980s developers of popular word-processing packages like WordStar and WordPerfect had incorporated spell checkers in their packages, mostly licensed from the above companies, who quickly expanded support from just English to European and eventually even Asian languages. However, this required increasing sophistication in the morphology routines of the software, particularly with regard to heavily-inflected languages like Hungarian and Finnish. Although the size of the word-processing market in a country like Iceland might not have justified the investment of implementing a spell checker, companies like WordPerfect nonetheless strove to localize their software for as many as possible national markets as part of their global marketing strategy.
Recently, spell checking has moved beyond word processors as Firefox 2.0, a web browser, has spell check support for user-written content, such as when editing Wikitext,writing on many webmail sites, blogs, and social networking websites. The web browsers Konqueror and Opera, the email client Kmail and the instant messaging client Pidgin also offer spell checking support, transparently using GNU Aspell as their engine.
Mac OS X now has spell check in virtually all bundled apps and many third party apple take advantage of this as well. Safari, Mail, iChat and more all have spell check capability.
Functionality The first spell checkers were "verifiers" instead of "correctors." They offered no suggestions for incorrectly spelled words. This was helpful for typos but it was not so helpful for logical or phonetic errors. The challenge the developers faced was the difficulty in offering useful suggestions for misspelled words. This requires reducing words to a skeletal form and applying pattern-matching algorithms.
It might seem logical that where spell-checking dictionaries are concerned, "the bigger, the better," so that correct words are not marked as incorrect. In practice, however, an optimal size for English appears to be around 90,000 entries. If there are more than this, incorrectly spelled words may be skipped because they are mistaken for others. For example, a linguist might determine in the basis of corpus linguistics that the word baht is more frequently a misspelling of bath or bat than a reference to the Thai currency. Hence, it would typically be more useful if a few people who write about Thai currency were slightly inconvenienced, than if the spelling errors of the many more people who discuss baths were overlooked.
The first MS-DOS spell checkers were mostly used in proofing mode from within word processing packages. After preparing a document, a user scanned the text looking for misspellings. Later, however, batch processing was offered in such packages as Oracle's short-lived CoAuthor. This allowed a user to view the results after a document was processed and only correct the words that he or she knew to be wrong. When memory and processing power became abundant, spell checking was performed in the background in an interactive way, such as has been the case with the Sector Software produced Spellbound program released in 1987 and Microsoft Word since Word 95.
In recent years, spell checkers have become increasingly sophisticated; some are now capable of recognizing simple grammatical errors. However, even at their best, they rarely catch all the errors in a text (such as homonym errors) and will flag neologisms and foreign words as misspelling.
Spell-checking other languages English is unusual in that most words used in formal writing have a single spelling that can be found in a typical dictionary, with the exception of some jargon and modified words. In many languages, however, it's typical to frequently combine words in new ways. In German, compound nouns are frequently coined from other existing nouns. Some scripts do not clearly separate one word from another, requiring word-splitting algorithms. Each of these presents unique challenges to non-English language spell checkers.
Context-sensitive spell checkersRecently, research has focused on developing algorithms which are capable of recognizing a misspelled word, even if the word itself is in the vocabulary, based on the context of the surrounding words . Not only does this allow words such as those in the poem above to be caught, but it mitigates the detrimental effect of enlarging dictionaries, allowing more words to be recognized. The most common example of errors caught by such a system are homophone errors, such as the bold words in the following sentence:
- Their coming too sea if its reel.
The most successful algorithm to date is Andrew Golding and Dan Roth's "Winnow-based spelling correction algorithm" , published in 1999, which is able to recognize about 96% of context-sensitive spelling errors, in addition to ordinary non-word spelling errors. A context-sensitive spell checker appears in Microsoft Office 2007.
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