OSTRICH

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.

Web Interface

For experiments, OSTRICH can also be used through the Princess web interface. Select the string solver option "OSTRICH" on the web interface for this purpose.

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)