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arXiv 1306.6054cs.LO
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具有长度和正则表达式约束的字方程的(Un)可判定性结果

(Un)Decidability Results for Word Equations with Length and Regular Expression Constraints

Vijay Ganesh, Mia Minnes, Armando Solar-Lezama, Martin Rinard

摘要:

我们证明了几个可判定性和不可判定性结果的可满足性和有效性问题的语言,可以表示解决方案的字方程的长度约束。这种语言上的原子公式是字符串项上的等式(单词等式)、长度函数上的线性不等式(长度约束)和正则集中的成员资格。这些问题在逻辑、程序分析和形式化验证中很重要。这些问题的变体已经被数学家研究了几十年。最近,这些公式的实际可满足性程序(又名SMT求解器)在字符串操作程序(如Web应用程序)的安全分析中变得越来越重要。我们证明了三个主要定理。首先,我们给出了一个新的证明不可判定的有效性问题的一组句子写为一个forall-exists量词交替应用于正字方程。这个不可判定性结果的一个推论是,这个集合是不可判定的,即使句子中最多出现两次字符串变量。第二,我们考虑布尔组合的量词自由公式构建的词方程和长度约束。我们表明,如果字方程可以转换为一个解决的形式,在实践中相关的形式,然后满足性问题的布尔组合的字方程和长度约束是可判定的。第三,我们证明了无量词公式在正则解形式、长度约束和正则表达式上的成员谓词上的可满足性问题也是可判定的。

英文摘要:

We prove several decidability and undecidability results for the satisfiability and validity problems for languages that can express solutions to word equations with length constraints. The atomic formulas over this language are equality over string terms (word equations), linear inequality over the length function (length constraints), and membership in regular sets. These questions are important in logic, program analysis, and formal verification. Variants of these questions have been studied for many decades by mathematicians. More recently, practical satisfiability procedures (aka SMT solvers) for these formulas have become increasingly important in the context of security analysis for string-manipulating programs such as web applications. We prove three main theorems. First, we give a new proof of undecidability for the validity problem for the set of sentences written as a forall-exists quantifier alternation applied to positive word equations. A corollary of this undecidability result is that this set is undecidable even with sentences with at most two occurrences of a string variable. Second, we consider Boolean combinations of quantifier-free formulas constructed out of word equations and length constraints. We show that if word equations can be converted to a solved form, a form relevant in practice, then the satisfiability problem for Boolean combinations of word equations and length constraints is decidable. Third, we show that the satisfiability problem for quantifier-free formulas over word equations in regular solved form, length constraints, and the membership predicate over regular expressions is also decidable.

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