Modern web applications often suffer from command injection attacks. Even when equipped with sanitization code, many systems can be penetrated due to software bugs. It is desirable to automatically discover such vulnerabilities, given the bytecode of a web application. One approach would be symbolically executing the target system and constructing constraints for matching path conditions and attack patterns. Solving these constraints yields an attack signature, based on which, the attack process can be replayed. Constraint solving is the key to symbolic execution. For web applications, string constraints receive most of the attention because web applications are essentially text processing programs. We present simple linear string equation (SISE) , a decidable fragment of the general string constraint system. SISE models a collection of regular replacement operations (such as the greedy, reluctant, declarative, and finite replacement), which are frequently used by text processing programs. Various automata techniques are proposed for simulating procedural semantics such as left-most matching. By composing atomic transducers of a SISE, we show that a recursive algorithm can be used to compute the solution pool, which contains the value range of each variable in concrete solutions. Then a concrete variable solution can be synthesized from a solution pool. To accelerate solver performance, a symbolic representation of finite state transducer is developed. This allows the constraint solver to support a 16-bit Unicode alphabet in practice. The algorithm is implemented in a Java constraint solver called SUSHI. We compare the applicability and performance of SUSHI with Kaluza, a bounded string solver.