propensive / kaleidoscope   0.5.0

Apache License 2.0 Website GitHub

Statically-checked inline matching on regular expressions in Scala

Scala versions: 2.13 2.12 2.11

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Statically-checked inline matching on regular expressions

Kaleidoscope is a small library which provides pattern matching using regular expressions, and extraction of capturing groups into values, which are typed according to the repetition of the group. Patterns can be written inline, directly in a case pattern, and do not need to be predefined.


  • pattern match strings against regular expressions
  • regular expressions can be written inline in patterns
  • extraction of capturing groups in patterns
  • typed extraction (into Lists or Options) of variable-length capturing groups
  • static verification of regular expression syntax
  • simpler "glob" syntax is also provided

Availability Plan

Kaleidoscope has not yet been published. The medium-term plan is to build Kaleidoscope with Fury and to publish it as a source build on Vent. This will enable ordinary users to write and build software which depends on Kaleidoscope.

Subsequently, Kaleidoscope will also be made available as a binary in the Maven Central repository. This will enable users of other build tools to use it.

For the overeager, curious and impatient, see building.

Getting Started

To use Kaleidoscope, first import its package,

import kaleidoscope.*

and you can then use a Kaleidoscope regular expression—a string prefixed with the letter r—anywhere you can use a pattern in Scala. For example,

import anticipation.Text

def describe(path: Text): Unit =
  path match
    case r"/images/.*" => println("image")
    case r"/styles/.*" => println("stylesheet")
    case _             => println("something else")


import vacuous.{Optional, Unset}

def validate(email: Text): Optional[Text] = email match
  case r"^[a-z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{2,6}$$" => email
  case _                                            => Unset

Such patterns will either match or not, however should they match, it is possible to extract parts of the matched string using capturing groups. The pattern syntax is exactly as described in the Java Standard Library, with the exception that a capturing group (enclosed within ( and )) may be bound to an identifier by placing it, like an interpolated string substitution, immediately prior to the capturing group, as $identifier or ${identifier}.

Here is an example:

enum FileType:
  case Image(text: Text)
  case Stylesheet(text: Text)

def identify(path: Text): FileType = path match
  case r"/images/${img}(.*)"  => FileType.Image(img)
  case r"/styles/$styles(.*)" => FileType.Stylesheet(styles)

Alternatively, this can be extracted directly in a val definition, like so:

val r"^[a-z0-9._%+-]+@$domain([a-z0-9.-]+\.$tld([a-z]{2,6}))$$" =
  "[email protected]": @unchecked

In the REPL, this would bind the following values:

> domain: Text = t""
> tld: Text = t"com"

In addition, the syntax of the regular expressionwill be checked at compile-time, and any issues will be reported then.

Repeated and optional capture groups

A normal, unitary capturing group will extract into a Text value. But if a capturing group has a repetition suffix, such as * or +, then the extracted type will be a List[Text]. This also applies to repetition ranges, such as {3}, {2,} or {1,9}. Note that {1} will still extract a Text value.

A capture group may be marked as optional, meaning it can appear either zero or one times. This will extract a value with the type Option[Text].

For example, see how init is extracted as a List[Text], below:

import gossamer.{drop, Rtl}

def parseList(): List[Text] = "parsley, sage, rosemary, and thyme" match
  case r"$only([a-z]+)"                      => List(only)
  case r"$first([a-z]+) and $second([a-z]+)" => List(first, second)
  case r"$init([a-z]+, )*and $last([a-z]+)"  =>, Rtl)) :+ last


Note that inside an extractor pattern string, whether it is single- (r"...") or triple-quoted (r"""..."""), special characters, notably \, do not need to be escaped, with the exception of $ which should be written as $$. It is still necessary, however, to follow the regular expression escaping rules, for example, an extractor matching a single opening parenthesis would be written as r"\(" or r"""\(""".


Kaleidoscope is classified as maturescent. For reference, Scala One projects are categorized into one of the following five stability levels:

  • embryonic: for experimental or demonstrative purposes only, without any guarantees of longevity
  • fledgling: of proven utility, seeking contributions, but liable to significant redesigns
  • maturescent: major design decisions broady settled, seeking probatory adoption and refinement
  • dependable: production-ready, subject to controlled ongoing maintenance and enhancement; tagged as version 1.0.0 or later
  • adamantine: proven, reliable and production-ready, with no further breaking changes ever anticipated

Projects at any stability level, even embryonic projects, can still be used, as long as caution is taken to avoid a mismatch between the project's stability level and the required stability and maintainability of your own project.

Kaleidoscope is designed to be small. Its entire source code currently consists of 519 lines of code.


Kaleidoscope will ultimately be built by Fury, when it is published. In the meantime, two possibilities are offered, however they are acknowledged to be fragile, inadequately tested, and unsuitable for anything more than experimentation. They are provided only for the necessity of providing some answer to the question, "how can I try Kaleidoscope?".

  1. Copy the sources into your own project

    Read the fury file in the repository root to understand Kaleidoscope's build structure, dependencies and source location; the file format should be short and quite intuitive. Copy the sources into a source directory in your own project, then repeat (recursively) for each of the dependencies.

    The sources are compiled against the latest nightly release of Scala 3. There should be no problem to compile the project together with all of its dependencies in a single compilation.

  2. Build with Wrath

    Wrath is a bootstrapping script for building Kaleidoscope and other projects in the absence of a fully-featured build tool. It is designed to read the fury file in the project directory, and produce a collection of JAR files which can be added to a classpath, by compiling the project and all of its dependencies, including the Scala compiler itself.

    Download the latest version of wrath, make it executable, and add it to your path, for example by copying it to /usr/local/bin/.

    Clone this repository inside an empty directory, so that the build can safely make clones of repositories it depends on as peers of kaleidoscope. Run wrath -F in the repository root. This will download and compile the latest version of Scala, as well as all of Kaleidoscope's dependencies.

    If the build was successful, the compiled JAR files can be found in the .wrath/dist directory.


Contributors to Kaleidoscope are welcome and encouraged. New contributors may like to look for issues marked beginner.

We suggest that all contributors read the Contributing Guide to make the process of contributing to Kaleidoscope easier.

Please do not contact project maintainers privately with questions unless there is a good reason to keep them private. While it can be tempting to repsond to such questions, private answers cannot be shared with a wider audience, and it can result in duplication of effort.


Kaleidoscope was designed and developed by Jon Pretty, and commercial support and training on all aspects of Scala 3 is available from Propensive OÜ.


Kaleidoscope is named after the optical instrument which shows pretty patterns to its user, while the library also works closely with patterns.

In general, Scala One project names are always chosen with some rationale, however it is usually frivolous. Each name is chosen for more for its uniqueness and intrigue than its concision or catchiness, and there is no bias towards names with positive or "nice" meanings—since many of the libraries perform some quite unpleasant tasks.

Names should be English words, though many are obscure or archaic, and it should be noted how willingly English adopts foreign words. Names are generally of Greek or Latin origin, and have often arrived in English via a romance language.


The logo is a loose allusion to a hexagonal pattern, which could appear in a kaleidoscope.


Kaleidoscope is copyright © 2024 Jon Pretty & Propensive OÜ, and is made available under the Apache 2.0 License.