Peggy

Peggy is a simple parser generator for JavaScript that produces fast parsers with excellent error reporting. You can use it to process complex data or computer languages and build transformers, interpreters, compilers and other tools easily.

Peggy is the successor of PEG.js which had been abandoned by its maintainer.

Migrating from PEG.js

Peggy version 1.x.x is API compatible with the most recent PEG.js release. Follow these steps to upgrade:

1. Uninstall pegjs (and @types/pegjs if you're using the DefinitelyTyped type definitions - we now include type definitions as part of peggy itself).
2. Replace all require("pegjs") or import ... from "pegjs" with require("peggy") or import ... from "peggy" as appropriate.
3. Any scripts that use the pegjs cli should now use peggy instead.
4. That's it!

Features

• Simple and expressive grammar syntax
• Integrates both lexical and syntactical analysis
• Parsers have excellent error reporting out of the box
• Based on parsing expression grammar formalism — more powerful than traditional LL(k) and LR(k) parsers
• Usable from your browser, from the command line, or via JavaScript API

Getting Started

Online version is the easiest way to generate a parser. Just enter your grammar, try parsing few inputs, and download generated parser code.

Installation

Node.js

To use the peggy command, install Peggy globally:

$ npm install -g peggy

To use the JavaScript API, install Peggy locally:

$ npm install peggy

If you need both the peggy command and the JavaScript API, install Peggy both ways.

Browser

The easiest way to use Peggy from the browser is to pull the latest version from a CDN. Either of these should work:

<script src="https://unpkg.com/peggy"></script>

<script src="https://cdn.jsdelivr.net/npm/peggy"></script>

When your document is done loading, there will be a global peggy object.

Generating a Parser

Peggy generates parser from a grammar that describes expected input and can specify what the parser returns (using semantic actions on matched parts of the input). Generated parser itself is a JavaScript object with a simple API.

Command Line

To generate a parser from your grammar, use the peggy command:

$ peggy arithmetics.peggy

This writes parser source code into a file with the same name as the grammar file but with “.js” extension. You can also specify the output file explicitly:

$ peggy -o arithmetics-parser.js arithmetics.peggy

If you omit both input and output file, standard input and standard output are used.

By default, the generated parser is in the Node.js module format. You can override this using the --format option.

You can tweak the generated parser with several options:

• --allowed-start-rules — comma-separated list of rules the parser will be allowed to start parsing from (default: the first rule in the grammar)
• --cache — makes the parser cache results, avoiding exponential parsing time in pathological cases but making the parser slower
• --dependency — makes the parser require a specified dependency (can be specified multiple times)
• --export-var — name of a global variable into which the parser object is assigned to when no module loader is detected
• --extra-options — additional options (in JSON format) to pass to peg.generate
• --extra-options-file — file with additional options (in JSON format) to pass to peg.generate
• --format — format of the generated parser: amd, es, commonjs, globals, umd (default: commonjs)
• --plugin — makes Peggy use a specified plugin (can be specified multiple times)
• --trace — makes the parser trace its progress

JavaScript API

In Node.js, require the Peggy parser generator module:

const peggy = require("peggy");

or

import * as peggy from "peggy";

For use in browsers, include the Peggy library in your web page or application using the <script> tag. If Peggy detects an AMD loader, it will define itself as a module, otherwise the API will be available in the peggy global object.

To generate a parser, call the peggy.generate method and pass your grammar as a parameter:

const parser = peggy.generate("start = ('a' / 'b')+");

The method will return generated parser object or its source code as a string (depending on the value of the output option — see below). It will throw an exception if the grammar is invalid. The exception will contain a message property with more details about the error.

You can tweak the generated parser by passing a second parameter with an options object to peg.generate. The following options are supported:

• allowedStartRules — rules the parser will be allowed to start parsing from (default: the first rule in the grammar)
• cache — if true, makes the parser cache results, avoiding exponential parsing time in pathological cases but making the parser slower (default: false)
• dependencies — parser dependencies, the value is an object which maps variables used to access the dependencies in the parser to module IDs used to load them; valid only when format is set to "amd", "commonjs", "es", or "umd". Dependencies variables will be available in both the global initializer and the per-parse initializer. Unless the parser is to be generated in different formats, it is recommended to rather import dependencies from within the global initializer. (default: {})
• exportVar — name of a global variable into which the parser object is assigned to when no module loader is detected; valid only when format is set to "globals" or "umd" (default: null)
• format — format of the generated parser ("amd", "bare", "commonjs", "es", "globals", or "umd"); valid only when output is set to "source" (default: "bare")
• grammarSource — this object will be passed to any location() objects as the source property (default: undefined). This object will be used even if options.grammarSource is redefined in the grammar. It is useful to attach the file information to the errors, for example
• output — if set to "parser", the method will return generated parser object; if set to "source", it will return parser source code as a string (default: "parser")
• plugins — plugins to use. See the Plugins API section
• trace — makes the parser trace its progress (default: false)

Using the Parser

To use the generated parser, call its parse method and pass an input string as a parameter. The method will return a parse result (the exact value depends on the grammar used to generate the parser) or throw an exception if the input is invalid. The exception will contain location, expected, found, message, and diagnostics properties with more details about the error. The error will have a format(SourceText[]) function, to which you pass an array of objects that look like { source: grammarSource, text: string }; this will return a nicely-formatted error suitable for human consumption.

parser.parse("abba"); // returns ["a", "b", "b", "a"]

parser.parse("abcd"); // throws an exception

You can tweak parser behavior by passing a second parameter with an options object to the parse method. The following options are supported:

• startRule — name of the rule to start parsing from
• tracer — tracer to use
• ... (any others) — made available in the options variable

As you can see above, parsers can also support their own custom options. For example:

const parser = peggy.generate(`
{
  // options are available in the per-parse initializer
  console.log(options.validWords);  // outputs "[ 'boo', 'baz', 'boop' ]"
}

validWord = @word:$[a-z]+ &{ return options.validWords.includes(word) }
`);

const result = parser.parse("boo", {
  validWords: [ "boo", "baz", "boop" ]
});

console.log(result);  // outputs "boo"

Grammar Syntax and Semantics

The grammar syntax is similar to JavaScript in that it is not line-oriented and ignores whitespace between tokens. You can also use JavaScript-style comments (// ... and /* ... */).

Let's look at example grammar that recognizes simple arithmetic expressions like 2*(3+4). A parser generated from this grammar computes their values.

start
  = additive

additive
  = left:multiplicative "+" right:additive { return left + right; }
  / multiplicative

multiplicative
  = left:primary "*" right:multiplicative { return left * right; }
  / primary

primary
  = integer
  / "(" additive:additive ")" { return additive; }

integer "integer"
  = digits:[0-9]+ { return parseInt(digits.join(""), 10); }

On the top level, the grammar consists of rules (in our example, there are five of them). Each rule has a name (e.g. integer) that identifies the rule, and a parsing expression (e.g. digits:[0-9]+ { return parseInt(digits.join(""), 10); }) that defines a pattern to match against the input text and possibly contains some JavaScript code that determines what happens when the pattern matches successfully. A rule can also contain human-readable name that is used in error messages (in our example, only the integer rule has a human-readable name). The parsing starts at the first rule, which is also called the start rule.

A rule name must be a JavaScript identifier. It is followed by an equality sign (“=”) and a parsing expression. If the rule has a human-readable name, it is written as a JavaScript string between the rule name and the equality sign. Rules need to be separated only by whitespace (their beginning is easily recognizable), but a semicolon (“;”) after the parsing expression is allowed.

The first rule can be preceded by a global initializer and/or a per-parse initializer, in that order. Both are pieces of JavaScript code in double curly braces (“{{” and “}}”) and single curly braces (“{” and “}”) respectively. All variables and functions defined in both initializers are accessible in rule actions and semantic predicates. Curly braces in both initializers code must be balanced.

The global initializer is executed once and only once, when the generated parser is loaded (through a require or an import statement for instance). It is the ideal location to require, to import, to declare constants, or to declare utility functions to be used in rule actions and semantic predicates.

The per-parse initializer is called before the generated parser starts parsing. The code inside the per-parse initializer can access the input string and the options passed to the parser using the input variable and the options variable respectively. It is the ideal location to create data structures that are unique to each parse or to modify the input before the parse.

Let's look at the example grammar from above using a global initializer and a per-parse initializer:

{{
  function makeInteger(o) {
    return parseInt(o.join(""), 10);
  }
}}

{
  if (options.multiplier) {
    input = "(" + input + ")*(" + options.multiplier + ")";
  }
}

start
  = additive

additive
  = left:multiplicative "+" right:additive { return left + right; }
  / multiplicative

multiplicative
  = left:primary "*" right:multiplicative { return left * right; }
  / primary

primary
  = integer
  / "(" additive:additive ")" { return additive; }

integer "integer"
  = digits:[0-9]+ { return makeInteger(digits); }

The parsing expressions of the rules are used to match the input text to the grammar. There are various types of expressions — matching characters or character classes, indicating optional parts and repetition, etc. Expressions can also contain references to other rules. See detailed description below.

If an expression successfully matches a part of the text when running the generated parser, it produces a match result, which is a JavaScript value. For example:

• An expression matching a literal string produces a JavaScript string containing matched text.
• An expression matching repeated occurrence of some subexpression produces a JavaScript array with all the matches.

The match results propagate through the rules when the rule names are used in expressions, up to the start rule. The generated parser returns start rule's match result when parsing is successful.

One special case of parser expression is a parser action — a piece of JavaScript code inside curly braces (“{” and “}”) that takes match results of some of the the preceding expressions and returns a JavaScript value. This value is considered match result of the preceding expression (in other words, the parser action is a match result transformer).

In our arithmetics example, there are many parser actions. Consider the action in expression digits:[0-9]+ { return parseInt(digits.join(""), 10); }. It takes the match result of the expression [0-9]+, which is an array of strings containing digits, as its parameter. It joins the digits together to form a number and converts it to a JavaScript number object.

Parsing Expression Types

There are several types of parsing expressions, some of them containing subexpressions and thus forming a recursive structure:

"literal"
'literal'

Match exact literal string and return it. The string syntax is the same as in JavaScript. Appending i right after the literal makes the match case-insensitive.

.

Match exactly one character and return it as a string.

[ characters ]

Match one character from a set and return it as a string. The characters in the list can be escaped in exactly the same way as in JavaScript string. The list of characters can also contain ranges (e.g. [a-z] means “all lowercase letters”). Preceding the characters with ^ inverts the matched set (e.g. [^a-z] means “all character but lowercase letters”). Appending i right after the right bracket makes the match case-insensitive.

rule

Match a parsing expression of a rule recursively and return its match result.

( expression )

Match a subexpression and return its match result.

expression *

Match zero or more repetitions of the expression and return their match results in an array. The matching is greedy, i.e. the parser tries to match the expression as many times as possible. Unlike in regular expressions, there is no backtracking.

expression +

Match one or more repetitions of the expression and return their match results in an array. The matching is greedy, i.e. the parser tries to match the expression as many times as possible. Unlike in regular expressions, there is no backtracking.

expression ?

Try to match the expression. If the match succeeds, return its match result, otherwise return null. Unlike in regular expressions, there is no backtracking.

& expression

Try to match the expression. If the match succeeds, just return undefined and do not consume any input, otherwise consider the match failed.

! expression

Try to match the expression. If the match does not succeed, just return undefined and do not consume any input, otherwise consider the match failed.

& { predicate }

This is a positive assertion. No input is consumed.

The predicate should be JavaScript code, and it's executed as a function. Curly braces in the predicate must be balanced.

The predicate should return a boolean value. If the result is truthy, it's match result is undefined, otherwise the match is considered failed.

The predicate has access to all variables and functions in the Action Execution Environment.

! { predicate }

This is a negative assertion. No input is consumed.

The predicate should be JavaScript code, and it's executed as a function. Curly braces in the predicate must be balanced.

The predicate should return a boolean value. If the result is falsy, it's match result is undefined, otherwise the match is considered failed.

The predicate has access to all variables and functions in the Action Execution Environment.

$ expression

Try to match the expression. If the match succeeds, return the matched text instead of the match result.

If you need to return the matched text in an action, use the text() function.

label : expression

Match the expression and remember its match result under given label. The label must be a JavaScript identifier, but not in the list of reserved words. By default this is a list of JavaScript reserved words, but plugins can change it.

Labeled expressions are useful together with actions, where saved match results can be accessed by action's JavaScript code.

@ ( label : )? expression

Match the expression and if the label exists, remember its match result under given label. The label must be a JavaScript identifier if it exists, but not in the list of reserved words. By default this is a list of JavaScript reserved words, but plugins can change it.

Return the value of this expression from the rule, or "pluck" it. You may not have an action for this rule. The expression must not be a semantic predicate (&{ predicate } or !{ predicate }). There may be multiple pluck expressions in a given rule, in which case an array of the plucked expressions is returned from the rule.

Pluck expressions are useful for writing terse grammars, or returning parts of an expression that is wrapped in parentheses.

expression₁ expression₂ ... expression_n

Match a sequence of expressions and return their match results in an array.

expression { action }

If the expression matches successfully, run the action, otherwise consider the match failed.

The action should be JavaScript code, and it's executed as a function. Curly braces in the action must be balanced.

The action should return some value, which will be used as the match result of the expression.

The action has access to all variables and functions in the Action Execution Environment.

expression₁ / expression₂ / ... / expression_n

Try to match the first expression, if it does not succeed, try the second one, etc. Return the match result of the first successfully matched expression. If no expression matches, consider the match failed.

Action Execution Environment

Actions and predicates have these variables and functions available to them.

• All variables and functions defined in the initializer or the top-level initializer at the beginning of the grammar are available.

• Note, that all functions and variables, described below, are unavailable in the global initializer.

• Labels from preceding expressions are available as local variables, which will have the match result of the labelled expressions.

  A label is only available after its labelled expression is matched:

  rule = A:('a' B:'b' { /* B is available, A is not */ } )
  

  A label in a sub-expression is only valid within the sub-expression:

  rule = A:'a' (B:'b') (C:'b' { /* A and C are available, B is not */ })
  

• input is a parsed string that was passed to the parse() method.

• options is a variable that contains the parser options. That is the same object that was passed to the parse() method.

• error(message, where) will report an error and throw an exception. where is optional; the default is the value of location().

• expected(message, where) is similar to error, but reports

  Expected message but "other" found.

  where other is, by default, the character in the location().start.offset position.

• location() returns an object with the information about the parse position. Refer to the corresponding section for the details.

• range() is similar to location(), but returns an object with offsets only. Refer to the "Locations" section for the details.

• offset() returns only the start offset, i.e. location().start.offset. Refer to the "Locations" section for the details.

• text() returns the source text between start and end (which will be "" for predicates). Instead of using that function as a return value for the rule consider using the $ operator.

Parsing Lists

One of the most frequent questions about Peggy grammars is how to parse a delimited list of items. The cleanest current approach is:

list = head:word tail:(_ "," _ @word)* { return [head, ...tail]; }
word = $[a-z]i+
_ = [ \t]*

Note that the @ in the tail section plucks the word out of the parentheses, NOT out of the rule itself.

Error Messages

As described above, you can annotate your grammar rules with human-readable names that will be used in error messages. For example, this production:

integer "integer"
  = digits:[0-9]+

will produce an error message like:

Expected integer but "a" found.

when parsing a non-number, referencing the human-readable name "integer." Without the human-readable name, Peggy instead uses a description of the character class that failed to match:

Expected [0-9] but "a" found.

Aside from the text content of messages, human-readable names also have a subtler effect on where errors are reported. Peggy prefers to match named rules completely or not at all, but not partially. Unnamed rules, on the other hand, can produce an error in the middle of their subexpressions.

For example, for this rule matching a comma-separated list of integers:

seq
  = integer ("," integer)*

an input like 1,2,a produces this error message:

Expected integer but "a" found.

But if we add a human-readable name to the seq production:

seq "list of numbers"
  = integer ("," integer)*

then Peggy prefers an error message that implies a smaller attempted parse tree:

Expected end of input but "," found.

There are two classes of errors in Peggy:

• SyntaxError: Syntax errors, found during parsing the input. This kind of errors can be thrown both during grammar parsing and during input parsing. Although name is the same, errors of each generated parser (including Peggy parser itself) has its own unique class.
• GrammarError: Grammar errors, found during construction of the parser. These errors can be thrown only in the parser generation phase. This error signals a logical mistake in the grammar, such as having two rules with the same name in one grammar, etc.

Both of these errors have the format() method that takes an array of mappings from source to grammar text:

let source = ...;
try {
  peggy.generate(text, { grammarSource: source, ... }); // throws SyntaxError or GrammarError
  parser.parse(input, { grammarSource: source2, ... }); // throws SyntaxError
} catch (e) {
  if (typeof e.format === "function") {
    console.log(e.format([
      { source, text },
      { source: source2, text: input },
      ...
    ]));
  } else {
    throw e;
  }
}

Messages generated by format() look like this:

Error: Possible infinite loop when parsing (left recursion: start -> proxy -> end -> start)
 --> .\recursion.pegjs:1:1
  |
1 | start = proxy;
  | ^^^^^
note: Step 1: call of the rule "proxy" without input consumption
 --> .\recursion.pegjs:1:9
  |
1 | start = proxy;
  |         ^^^^^
note: Step 2: call of the rule "end" without input consumption
 --> .\recursion.pegjs:2:11
  |
2 | proxy = a:end { return a; };
  |           ^^^
note: Step 3: call itself without input consumption - left recursion
 --> .\recursion.pegjs:3:8
  |
3 | end = !start
  |        ^^^^^

Locations

During the parsing you can access to the information of the current parse location, such as offset in the parsed string, line and column information. You can get this information by calling location() function, which returns you the following object:

{
  source: options.grammarSource,
  start: { offset: 23, line: 5, column: 6 },
  end: { offset: 25, line: 5, column: 8 }
}

source is an any object that was supplied in the grammarSource option in the parse() call. That object can be used to hold reference to the origin of the grammar, for example, it can be a filename. It is recommended that this object have a toString() implementation that returns meaningful string, because that string will be used when getting formatted error representation with e.format().

If source is null or undefined it doesn't appear in the formatted messages. The default value for source is undefined.

For actions, start refers to the position at the beginning of the preceding expression, and end refers to the position after the end of the preceding expression.

For semantic predicates, start and end are equal, denoting the location where the predicate is evaluated.

For the per-parse initializer, the location is the start of the input, i.e.

{
  source: options.grammarSource,
  start: { offset: 0, line: 1, column: 1 },
  end: { offset: 0, line: 1, column: 1 }
}

offset is a 0-based character index within the source text. line and column are 1-based indices.

The line number is incremented each time the parser finds an end of line sequence in the input.

Line and column are somewhat expensive to compute, so if you just need the offset, there's also a function offset() that returns just the start offset, and a function range() that returns the object:

{
  source: options.grammarSource,
  start: 23,
  end: 25
}

(i.e. difference from the location() result only in type of start and end properties, which contain just an offset instead of the Location object.)

All notes about values for location() object is also applicable to the range() and offset() calls.

Currently, Peggy only works with the Basic Multilingual Plane (BMP) of Unicode. This means that all offsets are measured in UTF-16 code units. If you try to parse characters outside this Plane (for example, emoji, or any surrogate pairs), you may get an offset inside a code point.

Changing this behavior may be a breaking change and will not to be done before Peggy 2.0. You can join to the discussion for this topic on the GitHub Discussions page.

Plugins API

A plugin is an object with the use(config, options) method. That method will be called for all plugins in the options.plugins array, supplied to the generate() method.

use accepts these parameters:

• config — object with the following properties:

  • parser — Parser object, by default the peggy.parser instance. That object will be used to parse the grammar. Plugin can replace this object

  • passes — mapping { [stage: string]: Pass[] } that represents compilation stages that would applied to the AST, returned by the parser object. That mapping will contain at least the following keys:

    • check — passes that check AST for correctness. They shouldn't change the AST
    • transform — passes that performs various optimizations. They can change the AST, add or remove nodes or their properties
    • generate — passes used for actual code generating

    A plugin that implement a pass usually should push it to the end of one of that arrays. Pass is a simple function with signature pass(ast, options):

    • ast — the AST created by the config.parser.parse() method
    • options — compilation options passed to the peggy.compiler.compile() method. If parser generation is started because generate() function was called that is also an options, passed to the generate() method

  • reservedWords — string array with a list of words that shouldn't be used as label names. This list can be modified by plugins. That property is not required to be sorted or not contain duplicates, but it is recommend to remove duplicates.

    Default list contains JavaScript reserved words, and can be found in the peggy.RESERVED_WORDS property.

• options — build options passed to the generate() method. A best practice for a plugin would look for its own options under a <plugin_name> key.

Compatibility

Both the parser generator and generated parsers should run well in the following environments:

• Node.js 10+
• Internet Explorer 9+
• Edge
• Firefox
• Chrome
• Safari
• Opera

Development

• Project website
• Wiki
• Source code
• Issue tracker
• Discussions

Peggy was originally developed by David Majda (@dmajda). It is currently maintained by Joe Hildebrand (@hildjj).

You are welcome to contribute code. Unless your contribution is really trivial you should get in touch with us first — this can prevent wasted effort on both sides.