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The magical sticky regex-based parser generator

Leveraging the power of sticky regexes and JS code generation, reghex allows you to code parsers quickly, by surrounding regular expressions with a regex-like DSL.

With reghex you can generate a parser from a tagged template literal, which is quick to prototype and generates reasonably compact and performant code.

This project is still in its early stages and is experimental. Its API may still change and some issues may need to be ironed out.

Quick Start

1. Install with yarn or npm
yarn add reghex
# or
npm install --save reghex
2. Add the plugin to your Babel configuration (optional)

In your .babelrc, babel.config.js, or package.json:babel add:

  "plugins": ["reghex/babel"]

Alternatively, you can set up babel-plugin-macros and import reghex from "reghex/macro" instead.

This step is optional. reghex can also generate its optimised JS code during runtime. This will only incur a tiny parsing cost on initialisation, but due to the JIT of modern JS engines there won't be any difference in performance between pre-compiled and compiled versions otherwise.

Since the reghex runtime is rather small, for larger grammars it may even make sense not to precompile the matchers at all. For this case you may pass the { "codegen": false } option to the Babel plugin, which will minify the reghex matcher templates without precompiling them.

3. Have fun writing parsers!
import { match, parse } from 'reghex';

const name = match('name')`

// [ "hello", .tag = "name" ]


The fundamental concept of reghex are regexes, specifically sticky regexes! These are regular expressions that don't search a target string, but instead match at the specific position they're at. The flag for sticky regexes is y and hence they can be created using /phrase/y or new RegExp('phrase', 'y').

Sticky Regexes are the perfect foundation for a parsing framework in JavaScript! Because they only match at a single position they can be used to match patterns continuously, as a parser would. Like global regexes, we can then manipulate where they should be matched by setting regex.lastIndex = index; and after matching read back their updated regex.lastIndex.

Note: Sticky Regexes aren't natively supported in any versions of Internet Explorer. reghex works around this by imitating its behaviour, which may decrease performance on IE11.

This primitive allows us to build up a parser from regexes that you pass when authoring a parser function, also called a "matcher" in reghex. When reghex compiles to parser code, this code is just a sequence and combination of sticky regexes that are executed in order!

let input = 'phrases should be parsed...';
let lastIndex = 0;

const regex = /phrase/y;
function matcher() {
  let match;
  // Before matching we set the current index on the RegExp
  regex.lastIndex = lastIndex;
  // Then we match and store the result
  if ((match = regex.exec(input))) {
    // If the RegExp matches successfully, we update our lastIndex
    lastIndex = regex.lastIndex;

This mechanism is used in all matcher functions that reghex generates. Internally reghex keeps track of the input string and the current index on that string, and the matcher functions execute regexes against this state.

Authoring Guide

You can write "matchers" by importing the match import from reghex and using it to write a matcher expression.

import { match } from 'reghex';

const name = match('name')`

As can be seen above, the match function, is called with a "node name" and is then called as a tagged template. This template is our parsing definition.

reghex functions only with its Babel plugin, which will detect match('name') and replace the entire tag with a parsing function, which may then look like the following in your transpiled code:

import { _pattern /* ... */ } from 'reghex';

var _name_expression = _pattern(/\w+/);
var name = function name() {
  /* ... */

We've now successfully created a matcher, which matches a single regex, which is a pattern of one or more letters. We can execute this matcher by calling it with the curried parse utility:

import { parse } from 'reghex';

const result = parse(name)('Tim');

console.log(result); // [ "Tim", .tag = "name" ]
console.log(result.tag); // "name"

If the string (Here: "Tim") was parsed successfully by the matcher, it will return an array that contains the result of the regex. The array is special in that it will also have a tag property set to the matcher's name, here "name", which we determined when we defined the matcher as match('name').

import { parse } from 'reghex';
parse(name)('42'); // undefined

Similarly, if the matcher does not parse an input string successfully, it will return undefined instead.

Nested matchers

This on its own is nice, but a parser must be able to traverse a string and turn it into an Abstract Syntax Tree. To introduce nesting to reghex matchers, we can refer to one matcher in another! Let's extend our original example;

import { match } from 'reghex';

const name = match('name')`

const hello = match('hello')`
  ${/hello /} ${name}

The new hello matcher is set to match /hello / and then attempts to match the name matcher afterwards. If either of these matchers fail, it will return undefined as well and roll back its changes. Using this matcher will give us nested abstract output.

We can also see in this example that outside of the regex interpolations, whitespace and newlines don't matter.

import { parse } from 'reghex';

parse(hello)('hello tim');
    ["tim", .tag = "name"],
    .tag = "hello"

Furthermore, interpolations don't have to just be RegHex matchers. They can also be functions returning matchers or completely custom matching functions. This is useful when your DSL becomes self-referential, i.e. when one matchers start referencing each other forming a loop. To fix this we can create a function that returns our root matcher:

import { match } from 'reghex';

const value = match('value')`
  (${/\w+/} | ${() => root})+

const root = match('root')`
  ${/root/}+ ${value}

Regex-like DSL

We've seen in the previous examples that matchers are authored using tagged template literals, where interpolations can either be filled using regexes, ${/pattern/}, or with other matchers ${name}.

The tagged template syntax supports more ways to match these interpolations, using a regex-like Domain Specific Language. Unlike in regexes, whitespace and newlines don't matter, which makes it easier to format and read matchers.

We can create sequences of matchers by adding multiple expressions in a row. A matcher using ${/1/} ${/2/} will attempt to match 1 and then 2 in the parsed string. This is just one feature of the regex-like DSL. The available operators are the following:

Operator Example Description
? ${/1/}? An optional may be used to make an interpolation optional. This means that the interpolation may or may not match.
* ${/1/}* A star can be used to match an arbitrary amount of interpolation or none at all. This means that the interpolation may repeat itself or may not be matched at all.
+ ${/1/}+ A plus is used like * and must match one or more times. When the matcher doesn't match, that's considered a failing case, since the match isn't optional.
| ${/1/} | ${/2/} An alternation can be used to match either one thing or another, falling back when the first interpolation fails.
() (${/1/} ${/2/})+ A group can be used to apply one of the other operators to an entire group of interpolations.
(?: ) (?: ${/1/}) A non-capturing group is like a regular group, but the interpolations matched inside it don't appear in the parser's output.
(?= ) (?= ${/1/}) A positive lookahead checks whether interpolations match, and if so continues the matcher without changing the input. If it matches, it's essentially ignored.
(?! ) (?! ${/1/}) A negative lookahead checks whether interpolations don't match, and if so continues the matcher without changing the input. If the interpolations do match the matcher is aborted.

A couple of operators also support "short hands" that allow you to write lookaheads or non-capturing groups a little quicker.

Shorthand Example Description
: :${/1/} A non-capturing group is like a regular group, but the interpolations matched inside it don't appear in the parser's output.
= =${/1/} A positive lookahead checks whether interpolations match, and if so continues the matcher without changing the input. If it matches, it's essentially ignored.
! !${/1/} A negative lookahead checks whether interpolations don't match, and if so continues the matcher without changing the input. If the interpolations do match the matcher is aborted.

We can combine and compose these operators to create more complex matchers. For instance, we can extend the original example to only allow a specific set of names by using the | operator:

const name = match('name')`
  ${/tim/} | ${/tom/} | ${/tam/}

parse(name)('tim'); // [ "tim", .tag = "name" ]
parse(name)('tom'); // [ "tom", .tag = "name" ]
parse(name)('patrick'); // undefined

The above will now only match specific name strings. When one pattern in this chain of alternations does not match, it will try the next one.

We can also use groups to add more matchers around the alternations themselves, by surrounding the alternations with ( and )

const name = match('name')`
  (${/tim/} | ${/tom/}) ${/!/}

parse(name)('tim!'); // [ "tim", "!", .tag = "name" ]
parse(name)('tom!'); // [ "tom", "!", .tag = "name" ]
parse(name)('tim'); // undefined

Maybe we're also not that interested in the "!" showing up in the output node. If we want to get rid of it, we can use a non-capturing group to hide it, while still requiring it.

const name = match('name')`
  (${/tim/} | ${/tom/}) (?: ${/!/})

parse(name)('tim!'); // [ "tim", .tag = "name" ]
parse(name)('tim'); // undefined

Lastly, like with regexes, ?, *, and + may be used as "quantifiers". The first two may also be optional and not match their patterns without the matcher failing. The + operator is used to match an interpolation one or more times, while the * operators may match zero or more times. Let's use this to allow the "!" to repeat.

const name = match('name')`
  (${/tim/} | ${/tom/})+ (?: ${/!/})*

parse(name)('tim!'); // [ "tim", .tag = "name" ]
parse(name)('tim!!!!'); // [ "tim", .tag = "name" ]
parse(name)('tim'); // [ "tim", .tag = "name" ]
parse(name)('timtim'); // [ "tim", tim", .tag = "name" ]

As we can see from the above, like in regexes, quantifiers can be combined with groups, non-capturing groups, or other groups.

Transforming as we match

In the previous sections, we've seen that the nodes that reghex outputs are arrays containing match strings or other nodes and have a special tag property with the node's type. We can change this output while we're parsing by passing a function to our matcher definition.

const name = match('name', (x) => x[0])`
  (${/tim/} | ${/tom/}) ${/!/}

parse(name)('tim'); // "tim"

In the above example, we're passing a small function, x => x[0] to the matcher as a second argument. This will change the matcher's output, which causes the parser to now return a new output for this matcher.

We can use this function creatively by outputting full AST nodes, maybe even like the ones that resemble Babel's output:

const identifier = match('identifier', (x) => ({
  type: 'Identifier',
  name: x[0],

parse(name)('var_name'); // { type: "Identifier", name: "var_name" }

We've now entirely changed the output of the parser for this matcher. Given that each matcher can change its output, we're free to change the parser's output entirely. By returning null or undefined in this matcher, we can also change the matcher to not have matched, which would cause other matchers to treat it like a mismatch!

import { match, parse } from 'reghex';

const name = match('name')((x) => {
  return x[0] !== 'tim' ? x : undefined;

const hello = match('hello')`
  ${/hello /} ${name}

parse(name)('tom'); // ["hello", ["tom", .tag = "name"], .tag = "hello"]
parse(name)('tim'); // undefined

Lastly, if we need to create these special array nodes ourselves, we can use reghex's tag export for this purpose.

import { tag } from 'reghex';

tag(['test'], 'node_name');
// ["test", .tag = "node_name"]

Tagged Template Parsing

Any grammar in RegHex can also be used to parse a tagged template literal. A tagged template literal consists of a list of literals alternating with a list of "interpolations".

In RegHex we can add an interpolation matcher to our grammars to allow it to parse interpolations in a template literal.

import { interpolation } from 'reghex';

const anyNumber = interpolation((x) => typeof x === 'number');

const num = match('num')`
  ${/[+-]?/} ${anyNumber}

// ["+", 42, .tag = "num"]

This grammar now allows us to match arbitrary values if they're input into the parser. We can now call our grammar using a tagged template literal themselves to parse this.

That's it! May the RegExp be ever in your favor.