Skip to content


Switch branches/tags


A parser generator based on Crystal and the UNIX philosophy. It is language agnostic, but can currently generate parsers for the C and Crystal languages.

Warning: Pegasus is experimental. Its APIs are not yet solidified, and are subject to change at any time.

Table of Contents


Pegasus is based on the UNIX philosophy of doing one thing, and doing it well. The core pegasus program isn't as much a parser generator as it is a Push Down Automaton generator.

Pegasus reads the grammar files, creates a Deterministic Finite Automaton (DFA) that is then used to tokenize (lex) text. Then, it creates an LALR(1) Push Down Automaton that is then used to parse text. However, it doesn't actually generate a parser: it outputs the generated tables for both automatons, as well as some extra information, as JSON. Another program, specific to each language, then reads the JSON and is responsible for code output.

This is beneficial because this prevents the code generator from being dependent on a language. JSON is a data interchange format, and it is easily readable from almost any programming language. If I, or others, want to add a code generation target, they can just parse the JSON in their preferred language, rather than Crystal. An additional benefit is that the addition of a target doesn't require the pegasus core to be updated or recompiled.


Pegasus parses grammars written in very basic notation. The grammars are separated into two sections: the tokens and the rules.


The tokens are terminals, and are described using regular expressions. An example token declaration is as follows:

token hello = /hello/;

Notice that the token declaration is terminated by a semicolon. Also notice that the regular expression is marked at both ends by a forward slash, /. In order to write a regular expression that includes a forward slash, it needs to be escaped, like \/. More information on regular expressions accepted by Pegasus can be found below.


Grammar rules appear after tokens in the grammar file. An example rule is given as follows:

rule S = hello;

This rule uses the token we declared above, that is, hello, which matches the string hello. In order to expect multiple tokens, we simply write them one after another:

rule S = hello hello;

Grammar rules aren't limited to only tokens. The names of other grammar rules, declared either earlier or later in the file, can also be used. For example:

rule S = two_hello hello;
rule two_hello = hello hello;

Here, we declare a second rule, two_hello, and then use it in the S rule.

Sometimes, it's useful to be able to declare several alternatives for rule. For example, we want to have an "operand" rule in a basic calculator, and an operand can either be a variable like "x" or a number like "3". We can write a rule as follows:

rule operand = number | variable;

A Note on Parse Trees

Earlier, we saw two rules written as follows:

rule S = two_hello hello;
rule two_hello = hello hello;

While it accepts the same language, this is not equivalent to the following:

rule S = hello hello hello;

The reason is that Pegasus, by default, produces parse trees. The first grammar will produce a parse tree whose root node, S, has two children, one being two_hello and the other being hello. The two_hello node will have two child nodes, both hello. However, the second variant will produce a parse tree whose root node, S, has three children, all hello.

Regular Expressions

Regular expressions support some basic operators:

  • hey+ matches hey, heyy, heyyy, and so on.
  • hey? matches hey or he
  • hey* matches he, hey, heyy, and so on.

Operators can also be applied to groups of characters:

  • (ab)+ matches ab, abab, ababab, and so on.

Please note, however, that Pegasus's lexer does not capture groups.


Pegasus supports an experimental mechanism to aid in parser generation, which involves attaching options to tokens or rules. Right now, the only option that is recognized is attached to a token definition. This option is "skip". Options are delcared as such:

token space = / +/ [ skip ];

The skip option means that the token it's attached to, in this case space, will be immediately discarded, and parsing will go on as if it wasn't there. For example, if we want a whitespace-insensitive list of digits, we can write it as such:

token space = / +/ [ skip ];
token digit = /[0-9]/;
token list_start = /\[/;
token list_end = /\]/;
token comma = /,/;

rule list = list_start list_recursive list_end;
rule list_recursive = digit | digit comma list_recursive;

Now, this will be able to parse equivalently the strings "[3]", "[ 3 ]" and [ 3]", because the whitespace token is ignored.

Semantic Actions

It's certainly convenient to create a parse tree that perfectly mimics the structure of a language's grammar. However, this isn't always desirable - if the user desires to construct an Abstract Syntax Tree, they're left having to walk the structure of the resulting tree again, frequently checking what rule created a particular nonterminal, or how many children a root node has. This is less than ideal - we don't want to duplicate the work of specifying the grammar when we walk the trees. Furthermore, if the grammar changes, the code that walks the parse trees will certainly need to change.

To remedy this, I've been toying with the idea of including semantic actions into Pegasus, in a very similar way to Yacc / Bison. Semantic actions are pieces of code that run when a particular rule in the grammar is matched. However, this would mean that the user has to write these actions in some particular language (Yacc / Bison use C/C++). Since Pegasus aims to be language agnostic, writing code in a particular language in the main grammar file is undesirable. Thus, I chose the approach of separating semantic actions into a separate file format. The format uses $$ to delimit code blocks, and contains the following sections:

  • Types that various nonterminals are assigned. For instance, a boolean expression can be assigned the C++ type "bool".
  • The actual rules that are of each of the types declared above.
  • The init code (placed in a global context before the parsing function)
  • The semantic actions for each rule.

For a concrete example of this file format, see the example code in the C Output With Semantic Actions section.

Included programs

Before you use any of these programs, you should use

shards build --release

This will compile all the Pegasus programs in release mode, for optimal performance.


This program reads grammars from standard input, and generates JSON descriptions out LALR automata, which will be read by the other programs. For example:

echo 'token hello = "Hello, world!"; rule S = hello;' > test.grammar
./bin/pegasus < test.grammar

This prints the JSON to the command line. If you'd like to output JSON to a file, you can use:

./bin/pegasus < test.grammar > test.json


This program is used largely for debugging purpose, and generates GraphViz DOT output, which can then by converted by the dot program into images. This greatly helps with debugging generated automata. pegasus-dot simply reads the generated JSON file:

./bin/pegasus-dot < test.json

To generate a PNG from the DOT output, you need the dot program installed. Once you have that, you can just pipe the output of pegasus-dot into dot:

./bin/pegasus-dot < test.json | dot -Tpng -o visual.png


This is another program largely used for debugging. Instead of generating a parser, it reads a JSON file, then attempts to parse text from STDIN. Once it's done, it prints the result of its attempt. Note that because it reads input from STDIN, rather than JSON, the JSON file has to be given as a command-line argument:

echo 'Hello, world!' | ./bin/pegasus-sim -i test.json


Finally, a parser generator! pegasus-c takes JSON, and creates C header and source files that can then be integrated into your project. To learn how to use the generated code, please take a look at the C output section.

./bin/pegasus-c < test.json


Another parser generator. pegasus-crystal outputs Crystal code which can then be integrated into your project. To learn how to use the generated code, lease take a look at the Crystal output section.

./bin/pegasus-crystal < test.json


Another C parser generator. The difference between this parser generator and pegasus-c is that it uses a separate semantic actions file to mimic the functionality of Yacc/Bison. This means you can specify C code that runs when each rule in the grammar is matched. To learn how to use this parser generator, see the C Output With Semantic Actions section.

./bin/pegasus-csem -l test.json -a test.sem

C Output

The pegasus repository contains the source code of a program that converts the JSON output into C source code. It generates a derivation tree, stored in pgs_tree, which is made up of nonterminal parent nodes and terminal leaves. Below is a simple example of using the functions generated for a grammar that describes the language of a binary operation applied to two numbers. The grammar:

token op_add = /\+/;
token op_sub = /-/;
token op_mul = /\*/;
token op_div = /\//;
token number = /[0-9]/;

rule S = expr;
rule expr = number op number;
rule op = op_add | op_sub | op_div | op_mul;

note: backslashes are necessary in the regular expressions because + and * are operators in the regular expression language.

The code for the API:

/* Include the generated header file */
#include "parser.h"
#include <stdio.h>

int main(int argc, char** argv) {
    pgs_state state; /* The state is used for reporting error messages.*/
    pgs_tree* tree; /* The tree that will be initialized */
    char buffer[256]; /* Buffer for string input */

    gets(buffer); /* Unsafe function for the sake of example */
    /* pgs_do_all lexes and parses the text from the buffer. */
    if(pgs_do_all(&state, &tree, buffer)) {
        /* A nonzero return code indicates error. Print it.*/
        printf("Error: %s\n", state.errbuff);
    } else {
        /* Do nothing, free the tree. */
        /* Tree is not initialized unless parse succeeds. */

This example is boring because nothing is done with the tree. Let's walk the tree and print it out:

void print_tree(pgs_tree* tree, const char* source, int indent) {
    size_t i;
    /* Print an indent. */
    for(i = 0; i < indent; i++) printf("  ");
    /* If the tree is a terminal (actual token) */
    if(tree->variant == PGS_TREE_TERMINAL) {
        printf("Terminal: %.*s\n", (int) (PGS_TREE_T_TO(*tree) - PGS_TREE_T_FROM(*tree)),
                source + PGS_TREE_T_FROM(*tree));
    } else {
        /* PGS_TREE_NT gives the nonterminal ID from the given tree. */
        printf("Nonterminal: %s\n", pgs_nonterminal_name(PGS_TREE_NT(*tree)));
        /* PGS_TREE_NT_COUNT returns the number of children a nonterminal
           node has. */
        for(i = 0; i < PGS_TREE_NT_COUNT(*tree); i++) {
            /* PGS_TREE_NT_CHILD gets the nth child of a nonterminal tree. */
            print_tree(PGS_TREE_NT_CHILD(*tree, i), source, indent + 1);

For the input string 3+3, the program will output:

Nonterminal: S
  Nonterminal: expr
    Nonterminal: number
      Terminal: 3
    Nonterminal: op
      Terminal: +
    Nonterminal: number
      Terminal: 3

Some more useful C macros for accessing the trees can be found in parser.h

C Output With Semantic Actions

Say you don't need a parse tree. Instead, you want to construct your own values from Pegasus grammar rules. In this case, you want to use the pegasus-csem parser generator. It is best demonstrated using a small example. Let's consider a language of booleans:

token whitespace = /[ \n\t]+/ [ skip ];
token true = /true/;
token false = /false/;
token and = /and/;
token or = /or/;

rule S = expr;
rule expr = tkn | expr and tkn | expr or tkn;
rule tkn = true | false;

Easy enough. But why would we want a parse tree from this? Let's operate directly on booleans (which we'll represent as integers in C). We create the semantic actions file step by step. First, we know all our actions will produce integers (which represent booleans). So we create a boolean type:

type boolean = $$ int $$

Now, we want to assign this type to the nonterminals in our language. We do this as follows:

typerules boolean = [ S, expr, tkn ]

We don't need any global variables or functions, so we can just leave the init block blank:

init = $$ $$

Next, we write actions that correspond to each gramamr rule.

rule S(0) = $$ $out = $0; $$

$out is the "output" variable, and $0 is the value generated for the first terminal or nonterminal in the rule (in this case, expr). This rule just forwards the result of the rules for expr. Next, let's write rules for expr:

rule expr(0) = $$ $out = $0; $$
rule expr(1) = $$ $out = $0 & $2; $$
rule expr(2) = $$ $out = $0 | $2; $$

The first rule simply forwards the value generated for tkn. The other two rules combine the results of their subexpressions using & and | (we use & in the grammar rule that has the and token, and | in the grammar rule that has the or token). Finally, we write the rules for tkn:

rule tkn(0) = $$ $out = 1; $$
rule tkn(1) = $$ $out = 0; $$

Time to test this. We need to write a simple program that uses the parser. The main difference from the C output without semantic actions is that we use pgs_stack_value union type, with fields named after the types we registered (boolean, in this case). The code:

#include "parser.h"

int main() {
    pgs_stack_value v; /* Temporary variable into which to store the result */
    pgs_state s; /* The state used for reporting error message */

    /* Initialize the state */
    /* Tokenize and parse a hardcoded string, ignoring error code */
    pgs_do_all(&s, &v, "false or false or true");
    /* Print the error generated, if any */
    printf("%s\n", s.errbuff);
    /* Print the boolean value as an integer. */
    printf("%d\n", v.boolean);

The output is the result of evaluating our expression: "true", or 1:


Crystal Output

Just like with C, this repository contains a program to output Crystal when code given a JSON file. Because Crystal supports exceptions and garbage collection, there is no need to initialize any variables, or call corresponding free functions. The most basic example of reading a line from the standard input and parsing it is below:

require "./"


Of course, this isn't particularly interesting. Let's add a basic function to print the tree:

def print_tree(tree, indent = 0)
  indent.times { STDOUT << "  " }
  case tree
  when Pegasus::Generated::TerminalTree
    STDOUT << "Terminal: "
    STDOUT.puts tree.string
  when Pegasus::Generated::NonterminalTree
    STDOUT << "Nonterminal: " <<
    tree.children.each { |it| print_tree(it, indent + 1) }

For the input string 3+3, the program will output:

Nonterminal: S
  Nonterminal: expr
    Nonterminal: number
      Terminal: 3
    Nonterminal: op
      Terminal: +
    Nonterminal: number
      Terminal: 3

Crystal Output with Semantic Actions

This is just like C semantic actions, but with Crystal. Suppose you don't need a parse tree. Rather, you want to generate your own values from Pegasus grammar rules. You can do this with the pegasus-crystalsem parser generator. When using this generator, you specify an additional file, which associates Crystal code (semantic actions) with each rule. Let's consider a language of booleans:

token whitespace = /[ \n\t]+/ [ skip ];
token true = /true/;
token false = /false/;
token and = /and/;
token or = /or/;

rule S = expr;
rule expr = tkn | expr and tkn | expr or tkn;
rule tkn = true | false;

Now that we have our grammar, it's time to formulate the additional file we mentioned. The first thing we need to do is figure out what Crystal type each of the nonterminals we generate. Our language is that of booleans, so we will be needing a boolean type:

type boolean = $$ Bool $$

Here, the stuff inside the $$ is Crystal code that is pasted verbatim into the generated parser. Now, we want to specify which rules evaluate to that type. In our simple language, every rule evaluates to a boolean:

typerules boolean = [ S, expr, tkn ]

pegasus-crystalsem also allows you to put some code above the parsing code, globally. We don't use this, so we leave the init property blank:

init = $$ $$

It is now time to assign semantic Crystal actions to each grammar rule. We start with the first rule, S(0) (which means the first rule for the S nonterminal). Since the first rule just matches an expr, we simply output the value of that expr:

rule S(0) = $$ $out = $0 $$

This means "set the output to be the value of the first element in the rule's body". We now implement the actual rules for expr. The first rule simply forwards the result of the tkn, just like the rule for S. The other two rules actually implement the logical operations of & and |:

rule expr(0) = $$ $out = $0 $$
rule expr(1) = $$ $out = $0 & $2 $$
rule expr(2) = $$ $out = $0 | $2 $$

Finally, we use the two rules for tkn to actually return a boolean:

rule tkn(0) = $$ $out = true $$
rule tkn(1) = $$ $out = false $$

Let's test this. We include the generated parser, and write the following:

require "./"

puts Pegasus::Generated.process(gets.not_nil!)

Let's now run this with the expression true or false or true. The output:


That's indeed our answer!

JSON Format

For the grammar given by:

token hi = /hi/;
rule A = hi;

The corresponding (pretty-printed) JSON output is:



  • DanilaFe Danila Fedorin - creator, maintainer


A parser generator for C and Crystal.








No releases published


No packages published