Jianhan Xu


2023

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Enhancing Unsupervised Semantic Parsing with Distributed Contextual Representations
Zixuan Ling | Xiaoqing Zheng | Jianhan Xu | Jinshu Lin | Kai-Wei Chang | Cho-Jui Hsieh | Xuanjing Huang
Findings of the Association for Computational Linguistics: ACL 2023

We extend a non-parametric Bayesian model of (Titov and Klementiev, 2011) to deal with homonymy and polysemy by leveraging distributed contextual word and phrase representations pre-trained on a large collection of unlabelled texts. Then, unsupervised semantic parsing is performed by decomposing sentences into fragments, clustering the fragments to abstract away syntactic variations of the same meaning, and predicting predicate-argument relations between the fragments. To better model the statistical dependencies between predicates and their arguments, we further conduct a hierarchical Pitman-Yor process. An improved Metropolis-Hastings merge-split sampler is proposed to speed up the mixing and convergence of Markov chains by leveraging pre-trained distributed representations. The experimental results show that the models achieve better accuracy on both question-answering and relation extraction tasks.

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Watermarking PLMs on Classification Tasks by Combining Contrastive Learning with Weight Perturbation
Chenxi Gu | Xiaoqing Zheng | Jianhan Xu | Muling Wu | Cenyuan Zhang | Chengsong Huang | Hua Cai | Xuanjing Huang
Findings of the Association for Computational Linguistics: EMNLP 2023

Large pre-trained language models (PLMs) have achieved remarkable success, making them highly valuable intellectual property due to their expensive training costs. Consequently, model watermarking, a method developed to protect the intellectual property of neural models, has emerged as a crucial yet underexplored technique. The problem of watermarking PLMs has remained unsolved since the parameters of PLMs will be updated when fine-tuned on downstream datasets, and then embedded watermarks could be removed easily due to the catastrophic forgetting phenomenon. This study investigates the feasibility of watermarking PLMs by embedding backdoors that can be triggered by specific inputs. We employ contrastive learning during the watermarking phase, allowing the representations of specific inputs to be isolated from others and mapped to a particular label after fine-tuning. Moreover, we demonstrate that by combining weight perturbation with the proposed method, watermarks can be embedded in a flatter region of the loss landscape, thereby increasing their robustness to watermark removal. Extensive experiments on multiple datasets demonstrate that the embedded watermarks can be robustly extracted without any knowledge about downstream tasks, and with a high success rate.

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Parameter Efficient Multi-task Fine-tuning by Learning to Transfer Token-wise Prompts
Muling Wu | Wenhao Liu | Jianhan Xu | Changze Lv | Zixuan Ling | Tianlong Li | Longtao Huang | Xiaoqing Zheng | Xuanjing Huang
Findings of the Association for Computational Linguistics: EMNLP 2023

Prompt tuning has been proven to be successful on various tasks by incorporating a small number of trainable parameters while freezing large pre-trained language models (PLMs). However, it is still unsettled how to generate more proper prompts for any individual examples and how to extend prompt tuning to multi-task learning scenarios by leveraging cross-task features. To address these challenges, we propose a token-wise prompt tuning (TPT), in which a bank of finer-grained soft prompt tokens is built for multi-task learning by memory network. The tokens are retrieved from the bank against an input example and assembled to an instance-dependent prompt. Extensive experimental results on 14 datasets demonstrated that the models enhanced by our TPT performed far better than full parameter fine-tuned models and achieved state-of-the-art by tuning only 0.035% parameters.

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Certified Robustness to Text Adversarial Attacks by Randomized [MASK]
Jiehang Zeng | Jianhan Xu | Xiaoqing Zheng | Xuanjing Huang
Computational Linguistics, Volume 49, Issue 2 - June 2023

Very recently, few certified defense methods have been developed to provably guarantee the robustness of a text classifier to adversarial synonym substitutions. However, all the existing certified defense methods assume that the defenders have been informed of how the adversaries generate synonyms, which is not a realistic scenario. In this study, we propose a certifiably robust defense method by randomly masking a certain proportion of the words in an input text, in which the above unrealistic assumption is no longer necessary. The proposed method can defend against not only word substitution-based attacks, but also character-level perturbations. We can certify the classifications of over 50% of texts to be robust to any perturbation of five words on AGNEWS, and two words on SST2 dataset. The experimental results show that our randomized smoothing method significantly outperforms recently proposed defense methods across multiple datasets under different attack algorithms.

2022

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Towards Adversarially Robust Text Classifiers by Learning to Reweight Clean Examples
Jianhan Xu | Cenyuan Zhang | Xiaoqing Zheng | Linyang Li | Cho-Jui Hsieh | Kai-Wei Chang | Xuanjing Huang
Findings of the Association for Computational Linguistics: ACL 2022

Most of the existing defense methods improve the adversarial robustness by making the models adapt to the training set augmented with some adversarial examples. However, the augmented adversarial examples may not be natural, which might distort the training distribution, resulting in inferior performance both in clean accuracy and adversarial robustness. In this study, we explore the feasibility of introducing a reweighting mechanism to calibrate the training distribution to obtain robust models. We propose to train text classifiers by a sample reweighting method in which the example weights are learned to minimize the loss of a validation set mixed with the clean examples and their adversarial ones in an online learning manner. Through extensive experiments, we show that there exists a reweighting mechanism to make the models more robust against adversarial attacks without the need to craft the adversarial examples for the entire training set.

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Weight Perturbation as Defense against Adversarial Word Substitutions
Jianhan Xu | Linyang Li | Jiping Zhang | Xiaoqing Zheng | Kai-Wei Chang | Cho-Jui Hsieh | Xuanjing Huang
Findings of the Association for Computational Linguistics: EMNLP 2022

The existence and pervasiveness of textual adversarial examples have raised serious concerns to security-critical applications. Many methods have been developed to defend against adversarial attacks for neural natural language processing (NLP) models. Adversarial training is one of the most successful defense methods by adding some random or intentional perturbations to the original input texts and making the models robust to the perturbed examples. In this study, we explore the feasibility of improving the adversarial robustness of NLP models by performing perturbations in the parameter space rather than the input feature space. The weight perturbation helps to find a better solution (i.e., the values of weights) that minimizes the adversarial loss among other feasible solutions. We found that the weight perturbation can significantly improve the robustness of NLP models when it is combined with the perturbation in the input embedding space, yielding the highest accuracy on both clean and adversarial examples across different datasets.

2021

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Searching for an Effective Defender: Benchmarking Defense against Adversarial Word Substitution
Zongyi Li | Jianhan Xu | Jiehang Zeng | Linyang Li | Xiaoqing Zheng | Qi Zhang | Kai-Wei Chang | Cho-Jui Hsieh
Proceedings of the 2021 Conference on Empirical Methods in Natural Language Processing

Recent studies have shown that deep neural network-based models are vulnerable to intentionally crafted adversarial examples, and various methods have been proposed to defend against adversarial word-substitution attacks for neural NLP models. However, there is a lack of systematic study on comparing different defense approaches under the same attacking setting. In this paper, we seek to fill the gap of systematic studies through comprehensive researches on understanding the behavior of neural text classifiers trained by various defense methods under representative adversarial attacks. In addition, we propose an effective method to further improve the robustness of neural text classifiers against such attacks, and achieved the highest accuracy on both clean and adversarial examples on AGNEWS and IMDB datasets by a significant margin. We hope this study could provide useful clues for future research on text adversarial defense. Codes are available at https://github.com/RockyLzy/TextDefender.

2020

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Cross-Lingual Dependency Parsing by POS-Guided Word Reordering
Lu Liu | Yi Zhou | Jianhan Xu | Xiaoqing Zheng | Kai-Wei Chang | Xuanjing Huang
Findings of the Association for Computational Linguistics: EMNLP 2020

We propose a novel approach to cross-lingual dependency parsing based on word reordering. The words in each sentence of a source language corpus are rearranged to meet the word order in a target language under the guidance of a part-of-speech based language model (LM). To obtain the highest reordering score under the LM, a population-based optimization algorithm and its genetic operators are designed to deal with the combinatorial nature of such word reordering. A parser trained on the reordered corpus then can be used to parse sentences in the target language. We demonstrate through extensive experimentation that our approach achieves better or comparable results across 25 target languages (1.73% increase in average), and outperforms a baseline by a significant margin on the languages that are greatly different from the source one. For example, when transferring the English parser to Hindi and Latin, our approach outperforms the baseline by 15.3% and 6.7% respectively.