Programming language: Crystal
Tags: Misc    

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Acorn Build Status

🚧 Under Construction 👷

A state machine compiler with no runtime dependency. Define a grammar using a subset of regular expression notation, then compile it into a blazing-fast state machine. Acorn supports lexers or custom string-based state machines.


Add this to your application's shard.yml:

    github: "rmosolgo/acorn"


  • Define the grammar in a build file:
  # ./build/my_lexer.cr
  require "acorn"

  class MyLexer < Acorn::Lexer
    # optional, rename the generated class:
    # name("Namespace::MyLexer")
    token :letter "a-z"
    token :number "0-9"
    generate "./src/my_lexer.cr"
  • Generate the lexer:
  crystal run ./build/my_lexer.cr
  • Use the compiled lexer:
  require "./src/my_lexer.cr"
  MyLexer.scan(input) # => Array(Tuple(Symbol, String))

Regular Expressions

Tokens are defined with a small regular expression language:

Feature Example
Character a, 1, ❤️
Sequence ab, 123
Alternation a|b
Grouping (ab)|c
Any character .
One of [abc]
Not one of [^abc]
Escape \[, \.
Unicode character range a-z, 0-9
Zero-or-more a*
One-or-more a+
Zero-or-one a?
Specific number a{3}
Between numbers a{3,4}
At least a{3,}

Build Step

An Acorn module is a Crystal program that generates code. To get a lexer, you have to run the Acorn module. Then, your main program should use the generated code.

For example, if you define a lexer:

# build/my_lexer.cr
class MyLexer < Acorn::Lexer
  # ...

You should run the file with Crystal to generate the specified file:

crystal run build/my_lexer.cr

Then, your main program should require the generated file:

# my_app.cr
require "app/my_lexer"
MyLexer.scan(input) # => Array(Tuple(Symbol, String))

The generated code has no dependency on Acorn, so you only need this library during development.


Acorn returns an of array tokens. Each token is a tuple with:

  • Symbol: the name of this token in the lexer definition
  • String: the segment of input which matched the pattern
  • {Int32, Int32}: line number and column number where the token began
  • {Int32, Int32}: line number and column number where the token ended

Line numbers and column numbers are 1-indexed, so the first character in the input is 1:1.

Custom Machines

Acorn lexers are actually a special case of state machine. You can specify a custom machine, too.

  • class MyMachine < Acorn::Machine to bring in the macros
  • alias Accumulator = ... to specify the data that will be modified during the process
    • It will be initialized with .new
    • It will be returned from .scan
  • use action :name, "pattern" { |acc, str, ts, te| ... } to define patterns
    • acc is an instance of your Accumulator
    • str is the original input
    • ts is the index in str where this token began
    • te is the index in str where this token ended (if the match is one character long, ts == te)
  • optionally, name("MyNamespace::MachineName") to rename the generated Crystal class


  • rebuild fixtures with crystal run spec/prepare.cr
  • crystal spec

Goals & Non-Goals


  • Great runtime performance
  • Linear time complexity
  • Reasonable code size (.cr file and memory usage)
  • No runtime dependency
  • Plain-Crystal API (source files are .cr, not a special format)
  • Easy to write a lexer, possible to write something else


  • Great compile-time performance (choose simplicity over performance)
  • Fancy regexp features


  • Finish regexp language ((...), ., [^...])
    • A move on a is also a move on :any, how is that handled?
  • Better line/col awareness:
    • Add line/col to tokens
    • Add line/col to errors



*Note that all licence references and agreements mentioned in the acorn README section above are relevant to that project's source code only.