A Morphographemic Model for Error Correction in Nonconcatenative Strings

A Morphographemic Model for Error Correction in Nonconcatenative Strings Tanya Bowden * and George Anton Kiraz t University of Cambridge Computer Laboratory Pembroke Street, Cambridge CB2 3QG {Tanya. Bowden, George.Kiraz}@cl. cam. ac. uk http://www, cl. cam. ac .uk/users/{tgblO00, gkl05} Abstract • This paper introduces a spelling correction system which integrates seamlessly with morphological analysis using a multi-tape formalism. Handling of various Semitic er- ror problems is illustrated, with reference to Arabic and Syriac examples. The model handles errors vocalisation, diacritics, pho- netic syncopation and morphographemic idiosyncrasies, in addition to Damerau er- rors. A complementary correction strategy for morphologically sound but morphosyn- tactically ill-formed words is outlined. 1 Introduction Semitic is known amongst computational linguists, in particular computational morphologists, for its highly inflexional morphology. Its root-and-pattern phenomenon not only poses difficulties for a mor- phological system, but also makes error detection a difficult task. This paper aims at presenting a morphographemic model which can cope with both issues. The following convention has been adopted. Mor- phemes are represented in braces, { }, surface (phonological) forms in solidi, //, and orthographic strings in acute brackets, (). In examples of gram- mars, variables begin with a capital letter. Cs de- note consonants, Vs denote vowels and a bar denotes complement. An asterisk, *, indicates ill-formed strings. The difficulties in morphological analysis and er- ror detection in Semitic arise from the following facts: * Supported by a British Telecom Scholarship, ad- ministered by the Cambridge Commonwealth Trust in conjunction with the Foreign sad Commonwealth Office. t Supported by a Benefactor Studentship from St John's College. Non-Linearity A Semitic stem consists of a root and a vowel melody, arranged accord- ing to a canonical pattern. For example, Arabic/kuttib/ 'caused to write - perfect pas- sive' is composed from the root morpheme {ktb} 'notion of writing' and the vowel melody morpheme {ul} 'perfect passive'; the two are arranged according to the pattern morpheme {CVCCVC} 'causative'. This phenomenon is analysed by (McCarthy, 1981) along the fines of autosegmental phonology (Goldsmith, 1976). The analysis appears in (1). 1 (1) DERIVATION OF /kuttib/ u i I I /kuttib/-- C V C C V C a v i k t b • Vocalisation Orthographically, Semitic texts appear in three forms: (i) consonantal texts do not incorporate any short vowels but ma- ttes lectionis, 2 e.g. Arabic (ktb) for /katab/, /kutib/and/kutub/, but (kaatb) for/kaatab/ and /kaatib/; (ii) partially voealised texts incorporate some short vowels to clarify am- biguity, e.g. (kutb) for /kutib/ to distinguish it from /katab/; and (iii) voealised texts in- corporate full vocalisation, e.g. (tadahra]) for /tada ay. 1We have used the CV model to describe pattern mor- phemes instead of prosodic terms because of its familiar- ity in the computational linguistics literature. For the use of moraic sad affLxational models in handling Arabic morphology computationally, see (Kiraz,). 2'Mothers of reading', these are consonantal letters which play the role of long vowels, sad are represented in the pattern morpheme by VV (e.g. /aa/, /uu/, /ii/). Mattes lectionis cannot be …

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