cpp-peglib

C++11 header-only PEG (Parsing Expression Grammars) library. You can start using it right away just by including peglib.h in your project.

You can also try the online version, PEG Playground at https://yhirose.github.io/cpp-peglib.

The PEG syntax is well described on page 2 in the document. cpp-peglib also supports the following additional syntax for now:

• '...'i (Case-insensitive literal operator)
• [^...] (Negated character class operator)
• {2,5} (Regex-like repetition operator)
• < ... > (Token boundary operator)
• ~ (Ignore operator)
• \x20 (Hex number char)
• %whitespace (Automatic whitespace skipping)
• %word (Word expression)
• $name( ... ) (Capture scope operator)
• $name< ... > (Named capture operator)
• $name (Backreference operator)
• | (Dictionary operator)
• MACRO_NAME( ... ) (Parameterized rule or Macro)
• { precedence L - + L / * } (Parsing infix expression)

This library supports the linear-time parsing known as the Packrat parsing.

IMPORTANT NOTE for some Linux distributions such as Ubuntu and CentOS: Need -pthread option when linking. See #23, #46 and #62.

How to use

This is a simple calculator sample. It shows how to define grammar, associate samantic actions to the grammar, and handle semantic values.

// (1) Include the header file
#include <peglib.h>
#include <assert.h>
#include <iostream>

using namespace peg;
using namespace std;

int main(void) {
    // (2) Make a parser
    parser parser(R"(
        # Grammar for Calculator...
        Additive    <- Multitive '+' Additive / Multitive
        Multitive   <- Primary '*' Multitive / Primary
        Primary     <- '(' Additive ')' / Number
        Number      <- < [0-9]+ >
        %whitespace <- [ \t]*
    )");

    assert((bool)parser == true);

    // (3) Setup actions
    parser["Additive"] = [](const SemanticValues& sv) {
        switch (sv.choice()) {
        case 0:  // "Multitive '+' Additive"
            return any_cast<int>(sv[0]) + any_cast<int>(sv[1]);
        default: // "Multitive"
            return any_cast<int>(sv[0]);
        }
    };

    parser["Multitive"] = [](const SemanticValues& sv) {
        switch (sv.choice()) {
        case 0:  // "Primary '*' Multitive"
            return any_cast<int>(sv[0]) * any_cast<int>(sv[1]);
        default: // "Primary"
            return any_cast<int>(sv[0]);
        }
    };

    parser["Number"] = [](const SemanticValues& sv) {
        return stoi(sv.token(), nullptr, 10);
    };

    // (4) Parse
    parser.enable_packrat_parsing(); // Enable packrat parsing.

    int val;
    parser.parse(" (1 + 2) * 3 ", val);

    assert(val == 9);
}

To show syntax errors in grammar text:

auto grammar = R"(
    # Grammar for Calculator...
    Additive    <- Multitive '+' Additive / Multitive
    Multitive   <- Primary '*' Multitive / Primary
    Primary     <- '(' Additive ')' / Number
    Number      <- < [0-9]+ >
    %whitespace <- [ \t]*
)";

parser parser;

parser.log = [](size_t line, size_t col, const string& msg) {
    cerr << line << ":" << col << ": " << msg << "\n";
};

auto ok = parser.load_grammar(grammar);
assert(ok);

There are four semantic actions available:

[](const SemanticValues& sv, any& dt)
[](const SemanticValues& sv)
[](SemanticValues& sv, any& dt)
[](SemanticValues& sv)

SemanticValues value contains the following information:

• Semantic values
• Matched string information
• Token information if the rule is literal or uses a token boundary operator
• Choice number when the rule is 'prioritized choise'

any& dt is a 'read-write' context data which can be used for whatever purposes. The initial context data is set in peg::parser::parse method.

peg::any is a simpler implementatin of std::any. If the compiler in use supports C++17, by default peg::any is defined as an alias to std::any.

To force using the simpler any implementation that comes with cpp-peglib, define PEGLIB_USE_STD_ANY as 0 before including peglib.h:

#define PEGLIB_USE_STD_ANY 0
#include <peglib.h>
[...]

A semantic action can return a value of arbitrary data type, which will be wrapped by peg::any. If a user returns nothing in a semantic action, the first semantic value in the const SemanticValues& sv argument will be returned. (Yacc parser has the same behavior.)

Here shows the SemanticValues structure:

struct SemanticValues : protected std::vector<any>
{
    // Input text
    const char* path;
    const char* ss;

    // Matched string
    std::string str() const;    // Matched string
    const char* c_str() const;  // Matched string start
    size_t      length() const; // Matched string length

    // Line number and column at which the matched string is
    std::pair<size_t, size_t> line_info() const;

    // Tokens
    std::vector<
        std::pair<
            const char*, // Token start
            size_t>>     // Token length
        tokens;

    std::string token(size_t id = 0) const;

    // Choice number (0 based index)
    size_t      choice() const;

    // Transform the semantic value vector to another vector
    template <typename T> vector<T> transform(size_t beg = 0, size_t end = -1) const;
}

The following example uses < ... > operator, which is token boundary operator.

auto syntax = R"(
    ROOT  <- _ TOKEN (',' _ TOKEN)*
    TOKEN <- < [a-z0-9]+ > _
    _     <- [ \t\r\n]*
)";

peg pg(syntax);

pg["TOKEN"] = [](const SemanticValues& sv) {
    // 'token' doesn't include trailing whitespaces
    auto token = sv.token();
};

auto ret = pg.parse(" token1, token2 ");

We can ignore unnecessary semantic values from the list by using ~ operator.

peg::parser parser(R"(
    ROOT  <-  _ ITEM (',' _ ITEM _)*
    ITEM  <-  ([a-z])+
    ~_    <-  [ \t]*
)");

parser["ROOT"] = [&](const SemanticValues& sv) {
    assert(sv.size() == 2); // should be 2 instead of 5.
};

auto ret = parser.parse(" item1, item2 ");

The following grammar is same as the above.

peg::parser parser(R"(
    ROOT  <-  ~_ ITEM (',' ~_ ITEM ~_)*
    ITEM  <-  ([a-z])+
    _     <-  [ \t]*
)");

Semantic predicate support is available. We can do it by throwing a peg::parse_error exception in a semantic action.

peg::parser parser("NUMBER  <-  [0-9]+");

parser["NUMBER"] = [](const SemanticValues& sv) {
    auto val = stol(sv.str(), nullptr, 10);
    if (val != 100) {
        throw peg::parse_error("value error!!");
    }
    return val;
};

long val;
auto ret = parser.parse("100", val);
assert(ret == true);
assert(val == 100);

ret = parser.parse("200", val);
assert(ret == false);

enter and leave actions are also avalable.

parser["RULE"].enter = [](const char* s, size_t n, any& dt) {
    std::cout << "enter" << std::endl;
};

parser["RULE"] = [](const SemanticValues& sv, any& dt) {
    std::cout << "action!" << std::endl;
};

parser["RULE"].leave = [](const char* s, size_t n, size_t matchlen, any& value, any& dt) {
    std::cout << "leave" << std::endl;
};

Ignoring Whitespaces

As you can see in the first example, we can ignore whitespaces between tokens automatically with %whitespace rule.

%whitespace rule can be applied to the following three conditions:

• trailing spaces on tokens
• leading spaces on text
• trailing spaces on literal strings in rules

These are valid tokens:

KEYWORD   <- 'keyword'
KEYWORDI  <- 'case_insensitive_keyword'
WORD      <-  < [a-zA-Z0-9] [a-zA-Z0-9-_]* >    # token boundary operator is used.
IDNET     <-  < IDENT_START_CHAR IDENT_CHAR* >  # token boundary operator is used.

The following grammar accepts one, "two three", four.

ROOT         <- ITEM (',' ITEM)*
ITEM         <- WORD / PHRASE
WORD         <- < [a-z]+ >
PHRASE       <- < '"' (!'"' .)* '"' >

%whitespace  <-  [ \t\r\n]*

Word expression

peg::parser parser(R"(
    ROOT         <-  'hello' 'world'
    %whitespace  <-  [ \t\r\n]*
    %word        <-  [a-z]+
)");

parser.parse("hello world"); // OK
parser.parse("helloworld");  // NG

Capture/Backreference

peg::parser parser(R"(
    ROOT      <- CONTENT
    CONTENT   <- (ELEMENT / TEXT)*
    ELEMENT   <- $(STAG CONTENT ETAG)
    STAG      <- '<' $tag< TAG_NAME > '>'
    ETAG      <- '</' $tag '>'
    TAG_NAME  <- 'b' / 'u'
    TEXT      <- TEXT_DATA
    TEXT_DATA <- ![<] .
)");

parser.parse("This is <b>a <u>test</u> text</b>."); // OK
parser.parse("This is <b>a <u>test</b> text</u>."); // NG
parser.parse("This is <b>a <u>test text</b>.");     // NG

Dictionary

| operator allows us to make a word dictionary for fast lookup by using Trie structure internally. We don't have to worry about the order of words.

START <- 'This month is ' MONTH '.'
MONTH <- 'Jan' | 'January' | 'Feb' | 'February' | '...'

Parameterized Rule or Macro

# Syntax
Start      ← _ Expr
Expr       ← Sum
Sum        ← List(Product, SumOpe)
Product    ← List(Value, ProOpe)
Value      ← Number / T('(') Expr T(')')

# Token
SumOpe     ← T('+' / '-')
ProOpe     ← T('*' / '/')
Number     ← T([0-9]+)
~_         ← [ \t\r\n]*

# Macro
List(I, D) ← I (D I)*
T(x)       ← < x > _

Parsing infix expression by Precedence climbing

Regarding the precedence climbing algorithm, please see this article.

parser parser(R"(
    EXPRESSION               <-  INFIX_EXPRESSION(ATOM, OPERATOR)
    ATOM                     <-  NUMBER / '(' EXPRESSION ')'
    OPERATOR                 <-  < [-+/*] >
    NUMBER                   <-  < '-'? [0-9]+ >
    %whitespace              <-  [ \t]*

    # Declare order of precedence
    INFIX_EXPRESSION(A, O) <-  A (O A)* {
      precedence
        L + -
        L * /
    }
)");

parser["INFIX_EXPRESSION"] = [](const SemanticValues& sv) -> long {
    auto result = any_cast<long>(sv[0]);
    if (sv.size() > 1) {
        auto ope = any_cast<char>(sv[1]);
        auto num = any_cast<long>(sv[2]);
        switch (ope) {
            case '+': result += num; break;
            case '-': result -= num; break;
            case '*': result *= num; break;
            case '/': result /= num; break;
        }
    }
    return result;
};
parser["OPERATOR"] = [](const SemanticValues& sv) { return *sv.c_str(); };
parser["NUMBER"] = [](const SemanticValues& sv) { return atol(sv.c_str()); };

long val;
parser.parse(" -1 + (1 + 2) * 3 - -1", val);
assert(val == 9);

precedence instruction can be applied only to the following 'list' style rule.

Rule <- Atom (Operator Atom)* {
  precedence
    L - +
    L / *
    R ^
}

precedence instruction contains precedence info entries. Each entry starts with associativity which is 'L' (left) or 'R' (right), then operator tokens follow. The first entry has the highest order level.

AST generation

cpp-peglib is able to generate an AST (Abstract Syntax Tree) when parsing. enable_ast method on peg::parser class enables the feature.

peg::parser parser("...");

parser.enable_ast();

shared_ptr<peg::Ast> ast;
if (parser.parse("...", ast)) {
    cout << peg::ast_to_s(ast);

    ast = peg::AstOptimizer(true).optimize(ast);
    cout << peg::ast_to_s(ast);
}

peg::AstOptimizer removes redundant nodes to make a AST simpler. You can make your own AST optimizers to fit your needs.

See actual usages in the AST calculator example and PL/0 language example.

Make a parser with parser combinators

Instead of makeing a parser by parsing PEG syntax text, we can also construct a parser by hand with parser combinatorss. Here is an example:

using namespace peg;
using namespace std;

vector<string> tags;

Definition ROOT, TAG_NAME, _;
ROOT     <= seq(_, zom(seq(chr('['), TAG_NAME, chr(']'), _)));
TAG_NAME <= oom(seq(npd(chr(']')), dot())), [&](const SemanticValues& sv) {
                tags.push_back(sv.str());
            };
_        <= zom(cls(" \t"));

auto ret = ROOT.parse(" [tag1] [tag:2] [tag-3] ");

The following are available operators:

Operator	Description
seq	Sequence
cho	Prioritized Choice
zom	Zero or More
oom	One or More
opt	Optional
apd	And predicate
npd	Not predicate
lit	Literal string
liti	Case-insensitive Literal string
cls	Character class
ncls	Negated Character class
chr	Character
dot	Any character
tok	Token boundary
ign	Ignore semantic value
csc	Capture scope
cap	Capture
bkr	Back reference
dic	Dictionary
pre	Infix expression
usr	User defined parser

Adjust definitions

It's possible to add/override definitions.

auto syntax = R"(
    ROOT <- _ 'Hello' _ NAME '!' _
)";

Rules additional_rules = {
    {
        "NAME", usr([](const char* s, size_t n, SemanticValues& sv, any& dt) -> size_t {
            static vector<string> names = { "PEG", "BNF" };
            for (const auto& name: names) {
                if (name.size() <= n && !name.compare(0, name.size(), s, name.size())) {
                    return name.size(); // processed length
                }
            }
            return -1; // parse error
        })
    },
    {
        "~_", zom(cls(" \t\r\n"))
    }
};

auto g = parser(syntax, additional_rules);

assert(g.parse(" Hello BNF! "));

Unicode support

cpp-peglib accepts UTF8 text. . matches a Unicode codepoint. Also, it supports \u????.

peglint - PEG syntax lint utility

Build peglint

> cd lint
> mkdir build
> cd build
> cmake ..
> make
> ./peglint
usage: peglint [--ast] [--optimize_ast_nodes|--opt] [--source text] [--trace] [grammar file path] [source file path]

Lint grammar

> cat a.peg
A <- 'hello' ^ 'world'

> peglint a.peg
a.peg:1:14: syntax error

> cat a.peg
A <- B

> peglint a.peg
a.peg:1:6: 'B' is not defined.

> cat a.peg
A <- B / C
B <- 'b'
C <- A

> peglint a.peg
a.peg:1:10: 'C' is left recursive.
a.peg:3:6: 'A' is left recursive.

Lint source text

> cat a.peg
Additive    <- Multitive '+' Additive / Multitive
Multitive   <- Primary '*' Multitive / Primary
Primary     <- '(' Additive ')' / Number
Number      <- < [0-9]+ >
%whitespace <- [ \t\r\n]*

> peglint --source "1 + a * 3" a.peg
[commendline]:1:3: syntax error

> cat a.txt
1 + 2 * 3

> peglint --ast a.peg a.txt
+ Additive
  + Multitive
    + Primary
      - Number (1)
  + Additive
    + Multitive
      + Primary
        - Number (2)
      + Multitive
        + Primary
          - Number (3)

> peglint --ast --opt --source "1 + 2 * 3" a.peg
+ Additive
  - Multitive[Number] (1)
  + Additive[Multitive]
    - Primary[Number] (2)
    - Multitive[Number] (3)

Sample codes

• Calculator
• Calculator (with parser operators)
• Calculator (AST version)
• Calculator (parsing expressions by precedence climbing)
• Calculator (AST version and parsing expressions by precedence climbing)
• PL/0 language example
• A tiny PL/0 JIT compiler in less than 700 LOC with LLVM and PEG parser
• A Programming Language just for writing Fizz Buzz program. :)

PEG debug

A debug viewer for Parsing Expression Grammars using cpp-peglib by mqnc. Please see his gihub project page for the detail. You can see a parse result of PL/0 code here.

License

MIT license (© 2020 Yuji Hirose)