Z3str3 is a constraint solver for the quantifier-free theory of string equations, the regular-expression membership predicates, and linear arithmetic over the length functions. Z3str3 is now part of the Z3 theorem prover’s main codebase, and is the primary string solver in Z3.

The Z3str3 string solver (theory_str) works with terms in the theory of strings with integer length and regular expressions. It also contains extensions for integration with bit-vector reasoning (e.g. C/C++ string operations), as well as native encodings of string library methods in other programming languages.