Issues in Arabic Search and Retrieval
With the exception of the Qur'an and pedagogical texts, Arabic is generally written without vowels or other graphic symbols that indicate how a word is pronounced. The reader is expected to fill these in from context. Some of the graphic symbols include sukuun, which is placed over a consonant to indicate that it is not followed by a vowel; shadda, written over a consonant to indicate it is doubled; and hamza, the sign of the glottal stop, which can be written above or below ا (alif) at the beginning of a word, or on ا (alif), و (waaw), ي (yaa'), or by itself on the line elsewhere. Also, common spelling differences regularly appear, including the use of ه (haa') for ة (taa' marbuuta) and ى (alif maqsuura) for ي (yaa'). These features of written Arabic, which are also seen in Hebrew as well as other languages written with Arabic script (such as Farsi, Pashto, and Urdu), make analyzing and searching texts quite challenging. In addition, Arabic morphology and grammar are quite rich and present some unique issues for information retrieval applications.
There are essentially three ways to search an Arabic text with Arabic queries: literal, stem-based or root-based.
A literal search, the simplest search and retrieval method, matches documents based on the search terms exactly as the user entered them. The advantage of this technique is that the documents returned will without a doubt contain the exact term for which the user is looking. But this advantage is also the biggest disadvantage: many, if not most, of the documents containing the terms in different forms will be missed. Given the many ambiguities of written Arabic, the success rate of this method is quite low. For example, if the user searches for كتاب (kitaab, book), he or she will not find documents that only contain ألكتاب (`al-kitaabu, the book).
Stem-based searching, a more complicated method, requires some normalization of the original texts and the queries. This is done by removing the vowel signs, unifying the hamza forms and removing or standardizing the other signs. Additionally, grammatical affixes and other constructions which attach directly to words, such as conjunctions, prepositions, and the definite article, should be identified and removed. Finally, regular and irregular plural forms need to be identified and reduced to their singular forms. Performing this type of stemming leads to more successful searches, but can be problematic due to over-generation or incorrect generation of stems.
A third method for searching Arabic texts is to index and search for the root forms of each word. Since most verbs and nouns in Arabic are derived from triliteral (or, rarely, quadriliteral) roots, identifying the underlying root of each word theoretically retrieves most of the documents containing a given search term regardless of form. However, there are some significant challenges with this approach. Determining the root for a given word is extremely difficult, since it requires a detailed morphological, syntactic and semantic analysis of the text to fully disambiguate the root forms. The issue is complicated further by the fact that not all words are derived from roots. For example, loan words (words borrowed from another language) are not based on root forms, although there are even exceptions to this rule. For example, some loans that have a structure similar to triliteral roots, such as the English word film, are handled grammatically as if they were root-based, adding to the complexity of this type of search. Finally, the root can serve as the foundation for a wide variety of words with related meanings. The root ك ت ب (ktb) is used for many words related to writing, including كتب (kataba, to write), كتاب (kitaab, book), مكتب (maktab, office), and كاتب (kaatib, author). But the same root is also used for regiment/battalion, كتيبة (katiiba). As a result, searching based on root forms results in very high recall, but precision is usually quite low.
While search and retrieval of Arabic text will never be an easy task, relying on linguistic analysis tools and methods can help make the process more successful. Ultimately, the search method you choose should depend on how critical it is to retrieve every conceivable instance of a word or phrase and the resources you have to process search returns in order to determine their true relevance.
This sidebar reprinted from #51 Volume 13 Issue 7 of MultiLingual Computing & Technology published by MultiLingual Computing, Inc., 319 North First Ave., Sandpoint, Idaho, USA, 208-263-8178, Fax: 208-263-6310.