Dynamic Entity-Based Named Entity Recognition Under Unconstrained Tagging Schemes

As increasingly more textual information becomes available, named entity recognition (NER) systems are thriving, benefiting from powerful models and expressive tagging schemes that promote the full use of diverse features at different levels. To improve performance, traditional approaches have focused mainly on changing the structures of NER models but have always ignored the hard constraints and left the NER tagging schemes unchanged. To solve this problem, this article proposes a dynamic entity-based NER approach under unconstrained tagging schemes. To eliminate the constraints, we reorganize widely used tagging schemes and propose two novel unconstrained schemes: one in which tags are assigned to words and entities separately, and one where words and entities are labeled indiscriminately by uniformly taking them as chunks. Associated with the unconstrained tagging schemes, two entity-based neural architectures are also presented that recognize entities at the same time that the sentence is dynamically segmented. Unlike other static NER models that process all the tags after labeling each word, our models address the inputs dynamically by the interactions between the input text and the output labels. The dynamic mechanism can ensure that the entity-level features are included in the NER system, which is helpful for correctly recognizing entities. Except for word embeddings pretrained from unlabeled corpora, no external language-specific knowledge or other resources such as gazetteers are used. The experiments with English, German, Dutch, and Spanish datasets show that our methods can perform very well with different languages. Particularly, the results of the recall rate against the entity’s length reveal that the proposed entity-based models are suitable for recognizing entities with long lengths.