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A lexical analyzer for JavaScript.


Version 2 is a complete rewrite with a new API, and requires a number of ES6 features. If you need compatibility with older browsers, the version 1 API is still supported.


npm install @desertnet/scanner


Let’s say you want to design a language for embedding things like images or videos in arbitrary text, using tags that look something like [img-1]. We can use this package to find those embedded tags.

import {createDialect, BufferedRegExpScanner} from '@desertnet/scanner'

const input = `Hi! [img-1]

First, declare the types of tokens you want to recognize.

const tag = Symbol('tag')
const text = Symbol('text')

Describe your language using regular expressions. String.raw can help you avoid needing to escape backslashes.

const r = String.raw
const embedTagLang = createDialect(
  [ tag,  r`\[\w-\d+\]` ],
  [ text, r`[^\[]+` ],
  [ text, r`\[` ],

Create a scanner to read the input string.

const embedTagScanner = new BufferedRegExpScanner(input)

Iterate over the tokens and do something with them.

for (const token of embedTagScanner.generateTokensUsingDialect(embedTagLang)) {
  const {type, start, end, line, column, value} = token
  console.log({type, start, end, line, column, value})

Here’s what that outputs.

{ type: Symbol(text), start: 0,  end: 4,  line: 1, column: 1,  value: 'Hi! ' }
{ type: Symbol(tag),  start: 4,  end: 11, line: 1, column: 5,  value: '[img-1]' }
{ type: Symbol(text), start: 11, end: 12, line: 1, column: 12, value: '\n' }
{ type: Symbol(tag),  start: 12, end: 19, line: 2, column: 1,  value: '[img-2]' }
{ type: Symbol(text), start: 19, end: 23, line: 2, column: 8,  value: '\nBye' }
{ type: Symbol(text), start: 23, end: 24, line: 3, column: 4,  value: '[' }
{ type: Symbol(text), start: 24, end: 26, line: 3, column: 5,  value: '!]' }


new BufferedRegExpScanner(inputString)

A Scanner subclass that scans a string using JavaScript RegExp patterns. “Buffered” refers to the fact that the entire input must be buffered into a string before scanning can start. It operates on the principle of “maximal munch”: if more than one possible TokenDefinition matches, the longest match is the token that is produced. In the event of a tie, the first TokenDefinition passed to Dialect is chosen amongst the longest matches.

Supported TokenDefinition flags:

  • ignoreCase: When set to true, acts like the i flag for regular expressions, causing the pattern to be case insensitive.

There are no specific properties for this Scanner subclass, see Scanner on how to use it to extract tokens from inputString.


This is a convenience function for creating a Dialect object with TokenDefinition objects. You pass it arrays of arguments to the TokenDefinition constructor, and a new Dialect object will be returned with those definitions.

new Dialect(tokenDefinitions)

A Dialect object is an ordered collection of TokenDefinition objects, passed as an array to this constructor. A dialect may be a complete language definition, or it may be a subset of a language.


An array of TokenDefinition objects that define the dialect.


The Scanner class should not be instantiated directly. Instead, instantiate a subclass like BufferedRegExpScanner.


Returns a JavaScript Generator that yields Tokens based on the passed Dialect. You can use this as the Iterable in a for...of loop.

In the event that no TokenDefinition in dialect matches, the generator will produce a final Token with a type property equal to the UnexpectedCharacter symbol and containing the start and end of the character. The UnexpectedCharacter symbol is exported by @desertnet/scanner. When this token is produced, the position property of scanner will not be updated.


Returns the line number for the passed offset of the input string.


Returns the column number for the passed offset of the input string.


This method should never be invoked by anything other than the Scanner class. However, if you are making a subclass of Scanner you must implement this method. The expected return value is an array with two values:

  1. Either the matching TokenDefinition object, or the identifier value of the TokenDefinition object. If possible, always return the definition object, as it allows for more flexible parsers.
  2. The offset within the input string where the token ends (the value of token.end).

If the end of the input is reached, it should return an array with the first value being that of the EOF symbol exported by @desertnet/scanner. If no token can be matched, undefined should be returned instead of an array.


The input string.


The offset into the input string that the scanner is currently evaluating.


The Token class should only be instantiated by the Scanner class.

Memory Usage

Instances of Token retain a reference to the Scanner that generated them, which will include a reference to the input string. This is so that properties like value, line and column can be computed on demand. However it does mean that to reclaim the memory used by the input string you must release any Token instances.


The token’s type identifier.


The offset into the input string where the token begins.


The offset into the input string where the token has ended. This is exclusive, so the offset of the last character of the token is actually token.end - 1. This is in keeping with how JavaScript string methods like .slice() work. It also allows for zero-length tokens to be represented, though it’s not clear if it is useful for a language to define tokens that can not have a length.


The string value of the token.


The line number that the token starts on. Note that this is a computed property, and the first access of it (or token.column) will trigger line number indexing of the input string. This means the first access of either line or column will be relatively slow, but subsequent accesses will be very fast.


The column number that the token starts on. Like line, this is a computed property; see token.line above for details.

new TokenDefinition(typeIdentifier, pattern, flags)

A TokenDefinition defines a mapping of a pattern to a token type identifier. In other words, you provide a pattern, and when the scanner matches that pattern in the input string, a Token object with its type property set to the provided typeIdentifier is generated.

  • typeIdentifier: A value used to identify the type of Token objects this definition produces. It can be of any type, but it is recommended that you use Symbols for efficiency and to prevent unintended naming collisions.
  • pattern: A string defining a pattern to be used by the scanner to match the input string. The format of this depends on the scanner implementation, but BufferedRegExpScanner expects JavaScript RegExp syntax.
  • flags: A plain object with flags for the scanner implementation. For example, to tell BufferedRegExpScanner to match case insensitively for this pattern, you would pass {ignoreCase: true}.


The passed typeIdentifier constructor parameter.


The passed pattern constructor parameter.


The passed flags constructor parameter.