snapdragon-lexer

Converts a string into an array of tokens, with useful methods for looking ahead and behind, capturing, matching, et cetera.

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Converts a string into an array of tokens, with useful methods for looking ahead and behind, capturing, matching, et cetera.

Please consider following this project's author, Jon Schlinkert, and consider starring the project to show your :heart: and support.

Table of Contents

Details
.set .get lexer.isLexer lexer.input lexer.string lexer.consumed lexer.tokens lexer.stash lexer.stack lexer.queue lexer.loc options.source options.mode options.value Plugin Conventions

Install

Install with npm:
$ npm install --save snapdragon-lexer

Breaking changes in v2.0!

Please see the changelog for details!

Usage

const Lexer = require('snapdragon-lexer');
const lexer = new Lexer();

lexer.capture('slash', /^\//);
lexer.capture('text', /^\w+/);
lexer.capture('star', /^\*/);

console.log(lexer.tokenize('foo/*'));

API

Lexer

Create a new Lexer with the given options.
Params
  • input {string|Object}: (optional) Input string or options. You can also set input directly on lexer.input after initializing.
  • options {object}

Example
const Lexer = require('snapdragon-lexer');
const lexer = new Lexer('foo/bar');

.bos

Returns true if we are still at the beginning-of-string, and no part of the string has been consumed.
  • returns {boolean}

.eos

Returns true if lexer.string and lexer.queue are empty.
  • returns {boolean}

.set

Register a handler function.
Params
  • type {string}
  • fn {function}: The handler function to register.

Example
lexer.set('star', function() {
  // do parser, lexer, or compiler stuff
});

.get

Get a registered handler function.
Params
  • type {string}
  • fn {function}: The handler function to register.

Example
lexer.set('star', function() {
  // do lexer stuff
});
const star = lexer.get('star');

.has

Returns true if the lexer has a registered handler of the given type.
Params
  • {string}: type
  • returns {boolean}

Example
lexer.set('star', function() {});
console.log(lexer.has('star')); // true

.token

Create a new Token with the given type and value.
Params
  • type {string|Object}: (required) The type of token to create
  • value {string}: (optional) The captured string
  • match {array}: (optional) Match results from String.match() or RegExp.exec()
  • returns {Object}: Returns an instance of snapdragon-token

Events
  • emits: token

Example
console.log(lexer.token({type: 'star', value: '*'}));
console.log(lexer.token('star', '*'));
console.log(lexer.token('star'));

.isToken

Returns true if the given value is a snapdragon-token instance.
Params
  • token {object}
  • returns {boolean}

Example
const Token = require('snapdragon-token');
lexer.isToken({}); // false
lexer.isToken(new Token({type: 'star', value: '*'})); // true

.consume

Consume the given length from lexer.string. The consumed value is used to update lexer.state.consumed, as well as the current position.
Params
  • len {number}
  • value {string}: Optionally pass the value being consumed.
  • returns {String}: Returns the consumed value

Example
lexer.consume(1);
lexer.consume(1, '*');

Returns a function for updating a token with lexer location information.
  • returns {function}

.match

Use the given regex to match a substring from lexer.string. Also validates the regex to ensure that it starts with ^ since matching should always be against the beginning of the string, and throws if the regex matches an empty string, which can cause catastrophic backtracking.
Params
  • regex {regExp}: (required)
  • returns {Array|null}: Returns the match array from RegExp.exec or null.

Example
const lexer = new Lexer('foo/bar');
const match = lexer.match(/^\w+/);
console.log(match);
//=> [ 'foo', index: 0, input: 'foo/bar' ]

.scan

Scan for a matching substring by calling .match() with the given regex. If a match is found, 1) a token of the specified type is created, 2) match[0] is used as token.value, and 3) the length of match[0] is sliced from lexer.string (by calling .consume()).
Params
  • type {string}
  • regex {regExp}
  • returns {Object}: Returns a token if a match is found, otherwise undefined.

Events
  • emits: scan

Example
lexer.string = '/foo/';
console.log(lexer.scan(/^\//, 'slash'));
//=> Token { type: 'slash', value: '/' }
console.log(lexer.scan(/^\w+/, 'text'));
//=> Token { type: 'text', value: 'foo' }
console.log(lexer.scan(/^\//, 'slash'));
//=> Token { type: 'slash', value: '/' }

.capture

Capture a token of the specified type using the provide regex for scanning and matching substrings. Automatically registers a handler when a function is passed as the last argument.
Params
  • type {string}: (required) The type of token being captured.
  • regex {regExp}: (required) The regex for matching substrings.
  • fn {function}: (optional) If supplied, the function will be called on the token before pushing it onto lexer.tokens.
  • returns {Object}

Example
lexer.capture('text', /^\w+/);
lexer.capture('text', /^\w+/, token => {
  if (token.value === 'foo') {
    // do stuff
  }
  return token;
});

.handle

Calls handler type on lexer.string.
Params
  • type {string}: The handler type to call on lexer.string
  • returns {Object}: Returns a token of the given type or undefined.

Events
  • emits: handle

Example
const lexer = new Lexer('/a/b');
lexer.capture('slash', /^\//);
lexer.capture('text', /^\w+/);
console.log(lexer.handle('text'));
//=> undefined
console.log(lexer.handle('slash'));
//=> { type: 'slash', value: '/' }
console.log(lexer.handle('text'));
//=> { type: 'text', value: 'a' }

.advance

Get the next token by iterating over lexer.handlers and calling each handler on lexer.string until a handler returns a token. If no handlers return a token, an error is thrown with the substring that couldn't be lexed.
  • returns {Object}: Returns the first token returned by a handler, or the first character in the remaining string if options.mode is set to character.

Example
const token = lexer.advance();

.lex

Tokenizes a string and returns an array of tokens.
Params
  • input {string}: The string to lex.
  • returns {Array}: Returns an array of tokens.

Example
let lexer = new Lexer({ handlers: otherLexer.handlers })
lexer.capture('slash', /^\//);
lexer.capture('text', /^\w+/);
const tokens = lexer.lex('a/b/c');
console.log(tokens);
// Results in:
// [ Token { type: 'text', value: 'a' },
//   Token { type: 'slash', value: '/' },
//   Token { type: 'text', value: 'b' },
//   Token { type: 'slash', value: '/' },
//   Token { type: 'text', value: 'c' } ]

.enqueue

Push a token onto the lexer.queue array.
Params
  • token {object}
  • returns {Object}: Returns the given token with updated token.index.

Example
console.log(lexer.queue.length); // 0
lexer.enqueue(new Token('star', '*'));
console.log(lexer.queue.length); // 1

.dequeue

Shift a token from lexer.queue.
  • returns {Object}: Returns the given token with updated token.index.

Example
console.log(lexer.queue.length); // 1
lexer.dequeue();
console.log(lexer.queue.length); // 0

.lookbehind

Lookbehind n tokens.
Params
  • n {number}
  • returns {Object}

Example
const token = lexer.lookbehind(2);

.prev

Get the previously lexed token.
  • returns {Object|undefined}: Returns a token or undefined.

Example
const token = lexer.prev();

.lookahead

Lookahead n tokens and return the last token. Pushes any intermediate tokens onto lexer.tokens. To lookahead a single token, use .peek().
Params
  • n {number}
  • returns {Object}

Example
const token = lexer.lookahead(2);

.peek

Lookahead a single token.
  • returns {Object}: Returns a token.

Example
const token = lexer.peek();

.next

Get the next token, either from the queue or by advancing.
  • returns {Object|String}: Returns a token, or (when options.mode is set to character) either gets the next character from lexer.queue, or consumes the next charcter in the string.

Example
const token = lexer.next();

.skip

Skip n tokens or characters in the string. Skipped values are not enqueued.
Params
  • n {number}
  • returns {Object}: returns an array of skipped tokens.

Example
const token = lexer.skip(1);

.skipWhile

Skip tokens while the given fn returns true.
Params
  • fn {function}: Return true if a token should be skipped.
  • returns {Array}: Returns an array if skipped tokens.

Example
lexer.skipWhile(tok => tok.type !== 'space');

.skipType

Skip the given token types.
Params
  • types {string|Array}: One or more token types to skip.
  • returns {Array}: Returns an array if skipped tokens.

Example
lexer.skipWhile(tok => tok.type !== 'space');

.skipType

Skip the given token types.
Params
  • types {string|Array}: One or more token types to skip.
  • returns {Array}: Returns an array if skipped tokens

Example
lexer.skipType('space');
lexer.skipType(['newline', 'space']);

.push

Pushes the given token onto lexer.tokens and calls .append() to push token.value onto lexer.stash. Disable pushing onto the stash by setting lexer.options.append or token.append to false.
Params
  • token {object|String}
  • returns {Object}: Returns the given token.

Events
  • emits: push

Example
console.log(lexer.tokens.length); // 0
lexer.push(new Token('star', '*'));
console.log(lexer.tokens.length); // 1
console.log(lexer.stash) // ['*']

.append

Append a string to the last element on lexer.stash, or push the string onto the stash if no elements exist.
Params
  • value {String}
  • returns {String}: Returns the last value in the array.

Example
const stack = new Stack();
stack.push('a');
stack.push('b');
stack.push('c');
stack.append('_foo');
stack.append('_bar');
console.log(stack);
//=> Stack ['a', 'b', 'c_foo_bar']

.isInside

Returns true if a token with the given type is on the stack.
Params
  • type {string}: The type to check for.
  • returns {boolean}

Example
if (lexer.isInside('bracket') || lexer.isInside('brace')) {
  // do stuff
}

.error

Throw a formatted error message with details including the cursor position.
Params
  • msg {string}: Message to use in the Error.
  • node {object}
  • returns {undefined}

Example
lexer.set('foo', function(tok) {
  if (tok.value !== 'foo') {
    throw this.state.error('expected token.value to be "foo"', tok);
  }
});

.use

Call a plugin function on the lexer instance.
Params
  • fn {function}
  • returns {object}: Returns the lexer instance.

Example
lexer.use(function(lexer) {
  // do stuff to lexer
});

Lexer#isLexer

Static method that returns true if the given value is an instance of snapdragon-lexer.
Params
  • lexer {object}
  • returns {Boolean}

Example
const Lexer = require('snapdragon-lexer');
const lexer = new Lexer();
console.log(Lexer.isLexer(lexer)); //=> true
console.log(Lexer.isLexer({})); //=> false

Lexer#isToken

Static method that returns true if the given value is an instance of snapdragon-token. This is a proxy to Token#isToken.
Params
  • lexer {object}
  • returns {Boolean}

Example
const Token = require('snapdragon-token');
const Lexer = require('snapdragon-lexer');
console.log(Lexer.isToken(new Token({type: 'foo'}))); //=> true
console.log(Lexer.isToken({})); //=> false

Lexer#State

The State class, exposed as a static property.

Lexer#Token

The Token class, exposed as a static property.

.set

Register a handler function.
Params
  • type {String}
  • fn {Function}: The handler function to register.

Example
lexer.set('star', function(token) {
  // do parser, lexer, or compiler stuff
});

As an alternative to .set, the .capture method will automatically register a handler when a function is passed as the last argument.

.get

Get a registered handler function.
Params
  • type {String}
  • fn {Function}: The handler function to register.

Example
lexer.set('star', function() {
  // do parser, lexer, or compiler stuff
});
const star = handlers.get('star');

Properties

lexer.isLexer

Type: {boolean}
Default: true (contant)
This property is defined as a convenience, to make it easy for plugins to check for an instance of Lexer.

lexer.input

Type: {string}
Default: ''
The unmodified source string provided by the user.

lexer.string

Type: {string}
Default: ''
The source string minus the part of the string that has already been consumed.

lexer.consumed

Type: {string}
Default: ''
The part of the source string that has been consumed.

lexer.tokens

Type: {array}
Default: `
Array of lexed tokens.

lexer.stash

Type: {array}
Default: [''] (instance of snapdragon-stack)
Array of captured strings. Similar to the lexer.tokens array, but stores strings instead of token objects.

lexer.stack

Type: {array}
Default: `
LIFO (last in, first out) array. A token is pushed onto the stack when an "opening" character or character sequence needs to be tracked. When the (matching) "closing" character or character sequence is encountered, the (opening) token is popped off of the stack.
The stack is not used by any lexer methods, it's reserved for the user. Stacks are necessary for creating Abstract Syntax Trees (ASTs), but if you require this functionality it would be better to use a parser such as snapdragon-parsersnapdragon-parser, with methods and other conveniences for creating an AST.

lexer.queue

Type: {array}
Default: `
FIFO (first in, first out) array, for temporarily storing tokens that are created when .lookahead() is called (or a method that calls .lookhead(), such as .peek()).
Tokens are dequeued when .next() is called.

lexer.loc

Type: {Object}
Default: { index: 0, column: 0, line: 1 }
The updated source string location with the following properties.
  • index - 0-index
  • column - 0-index
  • line - 1-index

The following plugins are available for automatically updating tokens with the location:

Options

options.source

Type: {string}
Default: undefined
The source of the input string. This is typically a filename or file path, but can also be 'string' if a string or buffer is provided directly.
If lexer.input is undefined, and options.source is a string, the lexer will attempt to set lexer.input by calling fs.readFileSync() on the value provided on options.source.

options.mode

Type: {string}
Default: undefined
If options.mode is character, instead of calling handlers (which match using regex) the .advance() method will consume and return one character at a time.

options.value

Type: {string}
Default: undefined
Specify the token property to use when the .push method pushes a value onto lexer.stash. The logic works something like this:
lexer.append(token[lexer.options.value || 'value']);

Tokens

See the snapdragon-token documentation for more details.

Plugins

Plugins are registered with the lexer.use() method and use the following conventions.

Plugin Conventions

Plugins are functions that take an instance of snapdragon-lexer.
However, it's recommended that you always wrap your plugin function in another function that takes an options object. This allow users to pass options when using the plugin. Even if your plugin doesn't take options, it's a best practice for users to always be able to use the same signature.
Example
function plugin(options) {
  return function(lexer) {
    // do stuff 
  };
}

lexer.use(plugin());

About

Contributing
Pull requests and stars are always welcome. For bugs and feature requests, please create an issue.
Please read the contributing guide for advice on opening issues, pull requests, and coding standards.

Running Tests
Running and reviewing unit tests is a great way to get familiarized with a library and its API. You can install dependencies and run tests with the following command:
$ npm install && npm test


Building docs
(This project's readme.md is generated by verb, please don't edit the readme directly. Any changes to the readme must be made in the .verb.md readme template.)
To generate the readme, run the following command:
$ npm install -g verbose/verb#dev verb-generate-readme && verb

Related projects

You might also be interested in these projects:

Author

Jon Schlinkert

License

Copyright © 2018, Jon Schlinkert. Released under the MIT License.

This file was generated by verb-generate-readme
, v0.8.0, on November 19, 2018.