Regular expressions (regexes) are a popular and powerful means of automatically manipulating text. Regexes are also an understudied denial of service vector (ReDoS). If a regex has super-linear worst-case complexity, an attacker may be able to trigger this complexity, exhausting the victim's CPU resources and causing denial of service. Existing research has shown how to detect these superlinear regexes, and practitioners have identified super-linear regex anti-pattern heuristics that may lead to such complexity.In this paper, we empirically study three major aspects of ReDoS that have hitherto been unexplored: the incidence of super-linear regexes in practice, how they can be prevented, and how they can be repaired. In the ecosystems of two of the most popular programming languages Ð JavaScript and Python ś we detected thousands of super-linear regexes affecting over 10,000 modules across diverse application domains. We also found that the conventional wisdom for super-linear regex anti-patterns has few false negatives but many false positives; these anti-patterns appear to be necessary, but not sufficient, signals of super-linear behavior. Finally, we found that when faced with a super-linear regex, developers favor revising it over truncating input or developing a custom parser, regardless of whether they had been shown examples of all three fix strategies. These findings motivate further research into ReDoS, since many modules are vulnerable to it and existing mechanisms to avoid it are insufficient. We believe that ReDoS vulnerabilities are a larger threat in practice than might have been guessed. łSome people, when confronted with a problem, think 'I know, I'll use regular expressions. ' Now they have two problems. ž śJamie Zawinski CCS CONCEPTS• Software and its engineering → Empirical software validation; Software libraries and repositories; • Security and privacy → Denial-of-service attacks;

This paper explores the extent to which regular expressions (regexes) are portable across programming languages. Many languages offer similar regex syntaxes, and it would be natural to assume that regexes can be ported across language boundaries. But can regexes be copy/pasted across language boundaries while retaining their semantic and performance characteristics? In our survey of 158 professional software developers, most indicated that they re-use regexes across language boundaries and about half reported that they believe regexes are a universal language. We experimentally evaluated the riskiness of this practice using a novel regex corpus Ð 537,806 regexes from 193,524 projects written in JavaScript, Java, PHP, Python, Ruby, Go, Perl, and Rust. Using our polyglot regex corpus, we explored the hitherto-unstudied regex portability problems: logic errors due to semantic differences, and security vulnerabilities due to performance differences. We report that developers' belief in a regex lingua franca is understandable but unfounded. Though most regexes compile across language boundaries, 15% exhibit semantic differences across languages and 10% exhibit performance differences across languages. We explained these differences using regex documentation, and further illuminate our findings by investigating regex engine implementations. Along the way we found bugs in the regex engines of JavaScript-V8, Python, Ruby, and Rust, and potential semantic and performance regex bugs in thousands of modules. CCS CONCEPTS • Software and its engineering → Reusability; • Social and professional topics → Software selection and adaptation.

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Regular expressions (regexes) are a powerful mechanism for solving string-matching problems. They are supported by all modern programming languages, and have been estimated to appear in more than a third of Python and JavaScript projects. Yet existing studies have focused mostly on one aspect of regex programming: readability. We know little about how developers perceive and program regexes, nor the difficulties that they face.In this paper, we provide the first study of the regex development cycle, with a focus on (1) how developers make decisions throughout the process, ( 2) what difficulties they face, and (3) how aware they are about serious risks involved in programming regexes. We took a mixed-methods approach, surveying 279 professional developers from a diversity of backgrounds (including top tech firms) for a high-level perspective, and interviewing 17 developers to learn the details about the difficulties that they face and the solutions that they prefer.In brief, regexes are hard. Not only are they hard to read, our participants said that they are hard to search for, hard to validate, and hard to document. They are also hard to master: the majority of our studied developers were unaware of critical security risks that can occur when using regexes, and those who knew of the risks did not deal with them in effective manners. Our findings provide multiple implications for future work, including semantic regex search engines for regex reuse and improved input generators for regex validation.

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