We develop an open-source large-scale finitestate morphological processing toolkit (AraComLex) for Modern Standard Arabic (MSA) distributed under the GPLv3 license. The morphological transducer is based on a lexical database specifically constructed for this purpose. In contrast to previous resources, the database is tuned to MSA, eliminating lexical entries no longer attested in contemporary use. The database is built using a corpus of 1,089,111,204 words, a pre-annotation tool, machine learning techniques, and knowledge-based pattern matching to automatically acquire lexical knowledge. Our morphological transducer is evaluated and compared to LDC's SAMA (Standard Arabic Morphological Analyser).