cpp-peglib

C++11 header-only PEG (Parsing Expression Grammars) library.

cpp-peglib tries to provide more expressive parsing experience in a simple way. This library depends on only one header file. So, you can start using it right away just by including peglib.h in your project.

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

• < ... > (Token boundary operator)
• ~ (Ignore operator)
• \x20 (Hex number char)
• $name< ... > (Named capture operator)
• $name (Backreference operator)
• %whitespace (Automatic whitespace skipping)
• %word (Word expression)

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

If you need a Go language version, please see go-peg.

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>

using namespace peg;
using namespace std;

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

    parser parser(syntax);

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

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

    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);
}

There are two semantic actions available:

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

const SemanticValues& sv 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 boost::any. It can wrap arbitrary data type.

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

parser["ROOT"] = [&](const auto& 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(
    "  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 auto& 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 = [](any& dt) {
    std::cout << "enter" << std::endl;
};

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

parser["RULE"].leave = [](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'
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

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
cls	Character class
chr	Character
dot	Any character
tok	Token boundary
ign	Ignore semantic value
cap	Capture character
bkr	Back reference

Unicode support

Since cpp-peglib only accepts 8 bits characters, it probably accepts UTF-8 text. But . matches only a byte, not a Unicode character. Also, it dosn't support \u????.

Sample codes

• Calculator
• Calculator (with parser operators)
• Calculator (AST version)
• PEG syntax Lint utility
• PL/0 language example
• A tiny PL/0 JIT compiler in less than 700 LOC with LLVM and PEG parser

Tested compilers

• Visual Studio 2017
• Visual Studio 2015
• Visual Studio 2013 with update 5
• Clang++ 5.0.1
• Clang++ 5.0
• Clang++ 4.0
• Clang++ 3.5
• G++ 5.4 on Ubuntu 16.04

IMPORTANT NOTE for Ubuntu: Need -pthread option when linking. See #23.

TODO

• Unicode support (. matches a Unicode char. \u????, \p{L})

License

MIT license (© 2018 Yuji Hirose)