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)
$name( ... ) (Capture scope operator)
$name< ... > (Named capture operator)
$name (Backreference operator)
MACRO_NAME( ... ) (Parameterized rule or Macro)

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>
#include <iostream>

using namespace peg;
using namespace std;

int main(void) {
    // (2) Make a parser
    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);

    // (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 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 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 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::pegparser parser(
    "  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(
    "  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'
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

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 > _

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 csc Capture scope cap Capture bkr Back reference 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] [--server [PORT]] [--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

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 .

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 and #46 .

TODO

Advanced Unicode support ( Unicode regular expressoin )

cpp-peglib

cpp-peglib

How to use

Ignoring Whitespaces

Word expression

Capture/Backreference

Parameterized Rule or Macro

AST generation

Make a parser with parser combinators

Adjust definitions

Unicode support

peglint - PEG syntax lint utility

Build peglint

Lint grammar

Lint source text

Sample codes

PEG debug

Tested compilers

TODO

License

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