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Tegral Niwen Lexer, part of the Niwen project, is a lexer framework with a primary focus on ease-of-use.

All public functions in Tegral Niwen are documented using KDoc: you should be able to see this documentation directly in your IDE.

Lexer structure

The main entry point for Niwen Lexer is the niwenLexer function, within which you can create states (identified by their state label), within which you can define matchers (which can output tokens and/or perform actions).

val lexer = niwenLexer { // <- Entrypoint
default state { // <- State
'.' isToken Tokens.DOT thenState States.IN_STRING // <- Matcher
// ^ ------------------ --------------------------
// Recognizer | |
// Action Additional action

inString state {
matches("[^\"]+") isToken Tokens.STRING_CONTENT
'.' isToken Tokens.QUOTE thenState default

If your lexer only contains one state, you can write it like this instead:

val lexer = niwenLexer {
state {
// ...


Running the lexer can be done using the tokenize function on the lexer object returned by the niwenLexer function.

When executing the lexer, Niwen will start in the default state, then try each defined matcher in the order they were written in the state block.

  • Once a matcher successfully matches, the lexer will execute the matcher's action, which can either be to ignore the match (via .ignore) or emit a token (via isToken). Additional actions are also performed at this point, such as switching to a different state (via thenState).
  • If a matcher fails to match, the lexer will try the next matcher. If there are no matchers left in the current state, the lexing process will fail with an exception.

Note that this means that precedence within a lexer state is defined by the order in which matchers are defined.

Once Niwen is done tokenizing (i.e. the entire input has been processed), it will return the list of tokens.

Token types

Produced tokens are identified via a token type, which can be any TokenType instance.

There are two recommended ways of creating token types:

tokenType function

For small projects, you can use tokenType("myToken") to create a token type with the provided name. The name is only used for debugging purposes. You can then use store these types in variables and use them as such:

val tDot = tokenType("dot")
val tGreeting = tokenType("greeting")

val lexer = niwenLexer {
state {
'.' isToken tDot
"hello" isToken tGreeting

Enum classes

Enum classes are recommended for larger lexers with more than about 3 token types. You can create an enum class that implements TokenType like so:

enum class Tokens : TokenType {
// ...

And use them in a lexer like so:

val lexer = niwenLexer {
state {
'.' isToken Tokens.DOT
',' isToken Tokens.COMMA
"hello" isToken Tokens.GREETING


Recognizers are used on the left hand-side of a matcher (i.e. the part before isToken or .ignore). Their role is to recognize a sequence of characters and, if found, trigger the remainder of the matcher.

There are two kinds of recognizers:

  • Regular recognizers (R), which are just instances of the TokenRecognizer interface. These are the actual implementations of the matchers and can be used directly in your lexer, but are usually created using special functions.
  • Pseudo-recognizers (PR), which are not actual instances of the TokenRecognizer interface, but are just plain Kotlin objects that Tegral Niwen will internally convert to proper recognizers.

Here are all of the available, built-in recognizers:

Strings (PR)

When using a string, the input will be match against the exact provided string. This matching is case-sensitive.

val tHello = tokenType()
val tGoodbye = tokenType()
val lexer = tegralNiwen {
state {
"Hello" isToken tHello
"Goodbye" isToken tGoodbye
" ".ignore
val tokens = lexer.tokenize("Hello Goodbye Hello")
// tokens = [Hello{tHello},Goodbye{tGoodbye},Hello{tHello}]

Characters (PR)

You can also use characters in the same way as strings. This matching is case-sensitive.

val tA = tokenType()
val tB = tokenType()
val lexer = niwenLexer {
state {
'a' isToken tA
'b' isToken tB
val tokens = lexer.tokenize("abba")
// tokens = [a{tA},b{tB},b{tB},a{tA}]

Character ranges (PR)

Character ranges (i.e. Kotlin's CharRange, which can be created with a..z) can be used to match a single character within the provided range. This matching is case-sensitive.

val tac = tokenType()
val tdf = tokenType()
val lexer = niwenLexer {
state {
'a'..'c' isToken tac
'd'..'f' isToken tdf
val tokens = lexer.tokenize("afAXebc")
// tokens = [a{tac},f{tdf},e{tdf},b{tac},c{tac}]

anyOf (R)

anyOf recognizes any of the provided strings, in a similar way to the String pseudo-recognizer.

The anyOf function takes in a vararg of strings:

anyOf("some string", "hi", "hello")

Here's a full example:

val thello = tokenType()
val tbonjour = tokenType()
val lexer = niwenLexer {
state {
anyOf("Hello", "Hi") isToken thello
anyOf("Bonjour", "Salut") isToken tbonjour
" ".ignore
val tokens = lexer.tokenize("Hello Bonjour Hi")
// tokens = [Hello{thello},Bonjour{tbonjour},Hi{thello}]
Using lists with anyOf

If you need to use a List instead of a vararg, you can do so as follows:

val myStrings = listOf("Hello", "Hey", "Hi").toTypedArray()

// Then, wherever you want to create the recognizer

Regex with matches (R)

You can use matches to recognize a sequence of characters using a regex pattern.

Use matches("my[Regex]") to do so. The pattern will be matched starting at the current position in the input.

val tNumber = tokenType()
val tWord = tokenType()
val lexer = niwenLexer {
state {
matches("\\d+") isToken tNumber
matches("\\w+") isToken tWord
" ".ignore
val tokens = lexer.tokenize("Hello 42 World 09")
// tokens = [Hello{tWord},42{tNumber},World{tWord},09{tNumber}]


Internally, matches uses Java's Pattern class (with Pattern.compile), but a few additional configurations are applied for you to make working with regexes in the lexer easier:

  • Look-behind ((?<=foobar) and (?<!foobar)) will correctly look before the current position (useTransparentBounds(true))

  • Anchors (^ and $) match the start and end of the input string (useAnchoringBounds(false))

  • Capturing groups are not used for anything at the moment. Please open an issue if you need to use groups in some form or another.

Repeated recognizers (R)

.repeated allows you to repeat a recognizer as many times as possible. For example, while 'a'..'z' will only match a single character, 'a'..'z'.repeated will match as many characters as possible within the provided range.

.repeated can be used on any recognizer. You can also specify a minimum and/or maximum amount of repetitions using the repeated(min, max) function. Both min and max are optional parameters and are set to null by default to indicate "no minimum/maximum".

val ta = tokenType()
val tb1 = tokenType()
val tb2 = tokenType()
val tc = tokenType()
val lexer = lixy {
state {
"a".repeated isToken ta
"b".repeated(max = 3) isToken tb1
"b".repeated(4, 6) isToken tb2
"c".repeated(2) isToken tc
val tokens = lexer.tokenize("abbaaaacccbbbb")
// tokens = [a{ta},bb{tb1},aaaa{ta},ccc{tc},bbbb{tb2}]