An SMT Solver for String Constraints

OSTRICH is an SMT solver for string constraints.

Using Ostrich

After installing the Scala Build tool (SBT), you can assemble a JAR file using sbt assembly. To run it, use either the ostrich script in the root folder, or ostrich-client. The latter transparently spins up a server that continuously serves requests from the client script; useful to avoid cold-starting the JVM if you are running many instances.

See ./ostrich -help for more options.

Input Format

OSTRICH accepts constraints written using the SMT-LIB theory of strings. At this point, most of the operators in the theory are supported, but inputs need to be straightline; see this paper for a definition.

In addition to the standardized SMT-LIB operators, OSTRICH can handle a number of further functions.

Additional string functions

Name Explanation
str.reverse Reverse a string

Unary transducers

Finite-state transducers are a general way to introduce further string functions. Examples of functions that can be represented as transducers are encoders, decoders, extraction of sub-strings, removal of white-space characters, etc.

Finite-state transducers can be defined as (mutually) recursive functions, see this file for an example.

It is also possible to use prioritised finite-state transducers: multiple outgoing transitions from a state can be given priorities, and the transducer will take the transition with highest priority that will lead to a successful run. See this file for an example.

Additional regular expression constructors

Name Explanation
re.from_ecma2020 Parse a regular expression in textual ECMAScript 2020 format (example)
re.from_ecma2020_flags Parse a regular expression in textual ECMAScript 2020 format, with a second argument to specify flags (example)
re.case_insensitive Make any regular expression case insensitive (example)

Handling of capture groups

OSTRICH can also process regular expressions that include capture groups, lazy quantifiers, and anchors, although this is more experimental. For this functionality, OSTRICH understands a number of additional regular expression operators:

Name Explanation
re.*? Non-greedy star: similar to re.* but matching as few characters as possible
re.+? Non-greedy plus
re.opt? Non-greedy option
(_ re.loop? a b) Non-greedy loop
(_ re.capture n) Capture group with index n
(_ re.reference n) Reference to the contents of capture group n
re.begin-anchor The anchor ^
re.end-anchor The anchor $

Such augmented regular expressions can be used in combination with several new string functions. Those functions support in particular capture groups and references in the replacement strings:

Name Explanation
(_ str.extract n) Extract the contents of the n'th capture group (example)
str.replace_cg Replace the first match of a regular expression (example)
str.replace_cg_all Replace all matches of a regular expression (example)