Kaleidoscope is a small library which provides pattern matching using regular expressions, and extraction of capturing groups into values. In particular, patterns are written inline, and do not need to be predefined.


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,

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


email match
  case r"^[a-z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{2,6}$$" => Some(email)
  case _                                            => None

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 prefixing the group with an @, and the identifier to extract to, which in standard Scala syntax, is written either as $identifier or ${identifier}.

Here is an example:

path match
  case r"/images/${img}@(.*)"  => Image(img)
  case r"/styles/$styles@(.*)" => Stylesheet(styles)

or as an extractor on a val, like so (using the Scala REPL):

val r"^[a-z0-9._%+-]+@$domain@([a-z0-9.-]+\.$tld@([a-z]{2,6})$$" = "test@example.com"
> domain: String = "example.com"
> tld: String = "com"

In addition, regular expressions will be checked at compile-time, and any issues will be reported then.


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 currently has no support for optional or repeated capturing groups.