An elegant armor-plated JavaScript lexer modelled after flex. Easily extensible to tailor to your need for perfection.

npm install lex
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An elegant armor-plated JavaScript lexer modelled after flex. Easily extensible to tailor to your need for perfection.


Lexer may be installed on node.js via the node package manager using the command npm install lex.

You may also install it on RingoJS using the command ringo-admin install aaditmshah/lexer.

You may install it as a component for web apps using the command component install aaditmshah/lexer.


Creating a lexer is as simple as instantiating the constructor Lexer.

var lexer = new Lexer;

After creating a lexer you may add rules to the lexer using the method addRule. The first argument to the function must be a RegExp object (the pattern to match). The second argument must be a function (the action to call when the pattern matches some text). The arguments passed to the function are the lexeme that was matched and all the substrings matched by parentheticals in the regular expression if any.

var lexer = new Lexer;

lexer.addRule(/[a-f\d]+/i, function (lexeme) {
    return "HEX";

After adding rules to the lexer you may set the property input of the lexer to any string that you wish to tokenize and then call the method lex. The function returns the first non undefined value returned by an action. Else it returns undefined if it scans the entire input string. On calling lex it starts scanning where it last left off. The addRule and setInput methods of the lexer support chaining.

var lines = 0;
var chars = 0;

(new Lexer).addRule(/\n/, function () {
}).addRule(/./, function () {
}).setInput("Hello World!").lex();

If the lexer can't match any pattern then it executes the default rule which matches the next character in the input string. The default action may be specified as an argument to the constructor. Setting the property reject on the this object in an action to true tells the lexer to reject the current rule and match the next best rule.

var row = 1;
var col = 1;

var lexer = new Lexer(function (char) {
    throw new Error("Unexpected character at row " + row + ", col " + col + ": " + char);

lexer.addRule(/\n/, function () {
    col = 1;
}, []);

lexer.addRule(/./, function () {
    this.reject = true;
}, []);

lexer.input = "Hello World!";


You may even specify start conditions for every rule as an optional third argument to the addRule method (which must be an array of unsigned integers). By default all rules are active in the initial state (i.e. 0). Odd start conditions are inclusive while even start conditions are exclusive. Rules with an empty array as the third argument are always active.

Integration with Jison

The generated lexer may be used as a custom scanner for Jison. Actions must return tokens and associated text must be made available in the property yytext on the object this from within the action.

var Parser = require("jison").Parser;
var Lexer = require("lex");

var grammar = {
    "bnf": {
        "expression" :[[ "e EOF",   "return $1;"  ]],
        "e" :[[ "NUMBER",  "$$ = Number(yytext);" ]]

var parser = new Parser(grammar);
var lexer = parser.lexer = new Lexer;

lexer.addRule(/\s+/, function () {});

lexer.addRule(/[0-9]+(?:\.[0-9]+)?\b/, function (lexeme) {
    this.yytext = lexeme;
    return "NUMBER";

lexer.addRule(/$/, function () {
    return "EOF";


Starting from v1.6.0 you can return multiple values from an action by returning an array. The elements of the array will be returned individually by the lex method. This allows you to implement features like python style indentation as follows:

var indent = [0];

var lexer = new Lexer;

lexer.addRule(/^[\t ]*/, function (lexeme) {
    var indentation = lexeme.length;

    if (indentation > indent[0]) {
        return "INDENT";

    var tokens = [];

    while (indentation < indent[0]) {

    if (tokens.length) return tokens;

Global Patterns

Sometimes you may wish to match a pattern which need not necessarily generate the longest possible string. Since the scanner sorts the matched strings according to their length there's no way to do so. Hence in v1.7.0 I introduced global patterns. Strings matching these patterns are never sorted. This allows you to match a shorter strings before longer ones:

var lexer = new Lexer;

lexer.addRule(/^ */gm, function (lexeme) {

lexer.addRule(/[0-9]+/, function (lexeme) {



The above program first logs the number of spaces at the beginning of the line (0) and then the number 37 although the length of the string "37" is greater than the empty string "". This is because it's the first rule and the global flag for its pattern is set.

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