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-typed inline pattern matching on regular expressions

Kaleidoscope is a small library to make pattern matching against strings more pleasant. Regular expressions can be written directly in patterns, and capturing groups bound directly to variables, typed according to the group's repetition. Here is an example:

case class Email(user: Text, domain: Text)

email match
  case r"$user([^@]+)@$domain(.*)" => Email(name, domain)

Strings are widely used to carry complex data, when it's wiser to use structured objects. Kaleidoscope makes it easier to move away from strings.


  • pattern match strings against regular expressions
  • regular expressions can be written inline in patterns, anywhere a string could match
  • direct extraction of capturing groups in patterns
  • typed extraction (into Lists or Vacuous Optionals) of variable-length capturing groups
  • static checking of regular expression syntax
  • simpler "glob" syntax is also provided


Kaleidoscope is available as a binary for Scala 3.4.0 and later, from Maven Central. To include it in an sbt build, use the coordinates:

libraryDependencies += "dev.soundness" % "kaleidoscope-core" % "0.1.0"

Getting Started

Kaleidoscope is included in the kaleidoscope package, and exported to the soundness package.

To use Kaleidoscope alone, you can include the import,

import kaleidoscope.*

or to use it with other Soundness libraries, include:

import soundness.*

Note that Kaleidoscope uses the Text type from Anticipation and the Optional type from Vacuous. These offer some advantages over String and Option, and they can be easily converted: Text#s converts a Text to a String and Optional#option converts an Optional value to its equivalent Option. The necessary imports are shown in the examples.

You can then use a Kaleidoscope regular expression—a string prefixed with the letter r—anywhere you can pattern match against a string 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 of using a pattern match against filenames:

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, as with patterns in general, this can be extracted directly in a val definition.

Here is an example of matching an email address:

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

The @unchecked annotation ascribed to the result is standard Scala, and acknowledges to the compiler that the match is partial and may fail at runtime.

If you try this example in the Scala REPL, it would bind the following values:

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

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

Repeated and optional capture groups

A normal, unitary capturing group, like domain and tld above, will extract into Text values. 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. The type is determined statically from the pattern, and not dynamically from the runtime scrutinee.

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 Optional[Text]; that is, if it present it will be a Text value, and if not, it will be Unset.

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"""\(""".


Globs offer a simplified and limited form of regular expression. You can use these in exactly the same way as a standard regular expresion, using the g"..." interpolator instead.


Kaleidoscope is classified as maturescent. For reference, Soundness 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 530 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, Soundness 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.