2.0.2 • Public • Published


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Simplified token definition and lexer creation library for use with the Chevrotain parser building toolkit.


const chevrotain = require('chevrotain')
const LD = require('lexdoc')(chevrotain)

const JsonLexer = LD.build({
  WhiteSpace: {
    pattern: /[ \t\n\r]+/,
    group: LD.SKIPPED,
    line_breaks: true

  NumberLiteral: /-?(0|[1-9]\d*)(\.\d+)?([eE][+-]?\d+)?/,
  StringLiteral: /"(?:[^\\"]|\\(?:[bfnrtv"\\/]|u[0-9a-fA-F]{4}))*"/,

  LCurly:  '{',
  RCurly:  '}',
  LSquare: '[',
  RSquare: ']',
  Comma:   ',',
  Colon:   ':',
  True:    'true',
  False:   'false',
  Null:    'null',

JsonLexer.instance // Stores the Chevrotain Lexer instance that was created

JsonLexer.lex(str) // Tokenize the given string. Wrapper around the created
                   // Lexer instance's #tokenize method, with built-in error
                   // handling.

JsonLexer.tokens        // An object containing the created tokens

JsonLexer.tokens.LCurly // Referencing a token


npm install lexdoc


Shortened Definitions for Simple Tokens

Tokens which have only a pattern and no other properties can be defined using only their pattern, simplifying many token definitions:

// Using the Chevrotain #createToken API:
const True = createToken({ name: "True", pattern: /true/ })
const False = createToken({ name: "False", pattern: /false/ })
const Null = createToken({ name: "Null", pattern: /null/ })
const LCurly = createToken({ name: "LCurly", pattern: /{/ })
const RCurly = createToken({ name: "RCurly", pattern: /}/ })

// Using Lexdoc
  True:   /true/,
  False:  /false/,
  Null:   /null/,
  LCurly: /{/,
  RCurly: /}/

Notice that the token name is not repeated using Lexdoc, keeping things DRY.

Soft Token References

References to other tokens within a token definition are done using a string representing the token's name, rather than a direct reference to the created token object. This has two advantages:

  • Tokens can be referenced before they are defined.
  • Tokens are defined in order of lexing precedence; order of precedence doesn't have to be specified separately.


  Boolean: LD.CATEGORY, // Categories are defined using LD.CATEGORY, a more
  Value:   LD.CATEGORY, // semantic synonym for Chevrotain's Lexer.NA

  // Reserved words
  True: {
    pattern: /true/,
    longer_alt: 'Identifier',   // Note that we reference Identifier before it's defined
    categories: 'Boolean Value'
  False: {
    pattern: /false/,
    longer_alt: 'Identifier',
    categories: 'Boolean Value' // Categories can be referenced using a
  },                            // space-seperated string. An array of strings
                                // could also be used.

  Identifier: {
    pattern: /[a-zA-Z][a-zA-Z_]*/,
    categories: 'Value'

Order of Definition is Order of Precedence

As seen above, the order of precedence when lexing is the same as the order in which tokens are defined when using Lexdoc. In Chevrotain tokens are defined and their order of precedence is then given separately.

Using Chevrotain:

// Identifier must be defined before Select or From so it can be referenced by
// them.
const Identifier = createToken({ name: "Identifier", pattern: /[a-zA-Z]\w*/ })

const Select = createToken({
    name: "Select",
    pattern: /SELECT/,
    longer_alt: Identifier
const From = createToken({
    name: "From",
    pattern: /FROM/,
    longer_alt: Identifier

// The order in which tokens will be lexed must be specified separately.
const tokens = [
  Identifier // Identifier must be lexed after Select and From

const lexer = new Lexer(tokens)

In Lexdoc token precedence during lexing is the same as the order in which tokens are defined:

const lexer = LD.build({
  Select: {
    pattern: /SELECT/,
    longer_alt: 'Identifier'
  From: {
    pattern: /FROM/,
    longer_alt: 'Identifier'
  Identifier: /[a-zA-Z]\w*/

Multi-mode Lexers

Multi-mode lexers are supported by Lexdoc:

LD.mode('ModeA', {
  TokenA: 'A',
  TokenB: 'B'
LD.mode('ModeB', {
  TokenC: 'C',
  TokenD: 'D'

LD.defaultMode('ModeB') // The first mode defined is implicitly set to be the
                        // default mode, but any other mode may be explicitly
                        // set as the default using #defaultMode

const lexer = LD.build()

Built-in XRegExp DSL

Lexdoc has a built-in XRegExp DSL, as seen in many of Chevrotain's examples, allowing for reuse of patterns.

Example usage:

// Define multiple fragments at once using #fragments
  fragA: 'foo',
  fragB: 'bar'

// Fragments can also be defined one at a time using #fragment
LD.fragment('fragC', 'baz')

// Fragments are re-used with the #pattern method
const lexer = LD.build({
  TokenA: LD.pattern('{{fragA}}.*?{{fragB}}'),
  TokenB: LD.pattern('\\({{fragC}}\\)')

Full Example

For a full example, see the provided JSON parser example.

Here's a stripped-down example showing only the important stuff:

const JsonLexer = LD.build({
  // Token definitions ...

class JsonParser extends Parser {
  constructor(input, config) {
    super(input, JsonLexer.tokens, config) // Use of lexer object's token list

    // Rules ...

    $.RULE('object', () => {
      $.CONSUME(JsonLexer.tokens.LCurly) // Reference to token
      // ...
      $.CONSUME(JsonLexer.tokens.RCurly) // Reference to token

    // Rules ...


const parser = new JsonParser(JsonLexer.tokens)

module.exports = function(text) {
  const lexResult = JsonLexer.lex(text) // Actual usage of the lexer
  parser.input = lexResult.tokens
  const value = parser.json()
  return value


Available under the terms of the MIT license.

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