cpp-peglib

C++17 header-only PEG (Parsing Expression Grammars) library. You can start using it right away just by including peglib.h in your project.

Since this library only supports C++17 compilers, please make sure that compiler the option -std=c++17 is enabled. (/std:c++17 /Zc:__cplusplus for MSVC)

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 by Bryan Ford. 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)
• \u10FFFF (Unicode char)
• %whitespace (Automatic whitespace skipping)
• %word (Word expression)
• $name( ... ) (Capture scope operator)
• $name< ... > (Named capture operator)
• $name (Backreference operator)
• | (Dictionary operator)
• ↑ (Cut operator)
• MACRO_NAME( ... ) (Parameterized rule or Macro)
• { precedence L - + L / * } (Parsing infix expression)
• %recovery( ... ) (Error recovery operator)
• exp⇑label or exp^label (Syntax sugar for (exp / %recover(label)))
• label { message "..." } (Error message instruction)

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 semantic 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(static_cast<bool>(parser) == true);

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

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

  parser["Number"] = [](const SemanticValues &vs) {
    return vs.token_to_number<int>();
  };

  // (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& vs, any& dt)
[](const SemanticValues& vs)
[](SemanticValues& vs, any& dt)
[](SemanticValues& vs)

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 choice'

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.

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& vs 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_view sv() const { return sv_; }

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

  // Tokens
  std::vector<std::string_view> tokens;
  std::string_view token(size_t id = 0) const;

  // Token conversion
  std::string token_to_string(size_t id = 0) const;
  template <typename T> T token_to_number() 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.

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

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

auto ret = parser.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& vs) {
  assert(vs.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& vs) {
  auto val = vs.token_to_number<long>();
  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 available.

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

parser["RULE"] = [](const SemanticValues& vs, 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' | '...'

Cut operator

↑ operator could mitigate backtrack performance problem, but has a risk to change the meaning of grammar.

S <- '(' ↑ P ')' / '"' ↑ P '"' / P
P <- 'a' / 'b' / 'c'

When we parse (z with the above grammar, we don't have to backtrack in S after ( is matched, because a cut operator is inserted there.

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& vs) -> long {
  auto result = any_cast<long>(vs[0]);
  if (vs.size() > 1) {
    auto ope = any_cast<char>(vs[1]);
    auto num = any_cast<long>(vs[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& vs) { return *vs.sv(); };
parser["NUMBER"] = [](const SemanticValues& vs) { return vs.token_to_number<long>(); };

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.

NOTE: An AST node holds a corresponding token as std::string_vew for performance and less memory usage. It is users' responsibility to keep the original source text along with the generated AST tree.

peg::parser parser(R"(
  ...
  defenition1 <- ... { no_ast_opt }
  defenition2 <- ... { no_ast_opt }
  ...
)");

parser.enable_ast();

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

  ast = parser.optimize_ast(ast);
  cout << peg::ast_to_s(ast);
}

optimize_ast removes redundant nodes to make a AST simpler. If you want to disable this behavior from particular rules, no_ast_opt instruction can be used.

It internally calls peg::AstOptimizer to do the job. 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 making a parser by parsing PEG syntax text, we can also construct a parser by hand with parser combinators. 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& vs) {
              tags.push_back(vs.token_to_string());
            };
_        <= zom(cls(" \t"));

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

The following are available operators:

Operator	Description	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
rec	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& vs, 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????.

Error report and recovery

cpp-peglib supports the furthest failure error position report as described in the Bryan Ford original document.

For better error report and recovery, cpp-peglib supports 'recovery' operator with label which can be associated with a recovery expression and a custom error message. This idea comes from the fantastic "Syntax Error Recovery in Parsing Expression Grammars" paper by Sergio Medeiros and Fabio Mascarenhas.

The custom message supports %t which is a place holder for the unexpected token, and %c for the unexpected Unicode char.

Here is an example of Java-like grammar:

# java.peg
Prog        ← 'public' 'class' NAME '{' 'public' 'static' 'void' 'main' '(' 'String' '[' ']' NAME ')' BlockStmt '}'
BlockStmt   ← '{' (!'}' Stmt^stmtb)* '}' # Annotated with `stmtb`
Stmt        ← IfStmt / WhileStmt / PrintStmt / DecStmt / AssignStmt / BlockStmt
IfStmt      ← 'if' '(' Exp ')' Stmt ('else' Stmt)?
WhileStmt   ← 'while' '(' Exp^condw ')' Stmt # Annotated with `condw`
DecStmt     ← 'int' NAME ('=' Exp)? ';'
AssignStmt  ← NAME '=' Exp ';'^semia # Annotated with `semi`
PrintStmt   ← 'System.out.println' '(' Exp ')' ';'
Exp         ← RelExp ('==' RelExp)*
RelExp      ← AddExp ('<' AddExp)*
AddExp      ← MulExp (('+' / '-') MulExp)*
MulExp      ← AtomExp (('*' / '/') AtomExp)*
AtomExp     ← '(' Exp ')' / NUMBER / NAME

NUMBER      ← < [0-9]+ >
NAME        ← < [a-zA-Z_][a-zA-Z_0-9]* >

%whitespace ← [ \t\n]*
%word       ← NAME

# Recovery operator labels
semia       ← '' { message "missing semicolon in assignment." }
stmtb       ← (!(Stmt / 'else' / '}') .)* { message "invalid statement" }
condw       ← &'==' ('==' RelExp)* / &'<' ('<' AddExp)* / (!')' .)*

For instance, ';'^semi is a syntactic sugar for (';' / %recovery(semi)). %recover operator tries to recover the error at ';' by skipping input text with the recovery expression semi. Also semi is associated with a custom message "missing semicolon in assignment.".

Here is the result:

> cat sample.java
public class Example {
  public static void main(String[] args) {
    int n = 5;
    int f = 1;
    while( < n) {
      f = f * n;
      n = n - 1
    };
    System.out.println(f);
  }
}

> peglint java.peg sample.java
sample.java:5:12: syntax error, unexpected '<', expecting <NAME>, <NUMBER>, <WhileStmt>.
sample.java:8:5: missing semicolon in assignment.
sample.java:8:6: invalid statement

As you can see, it can now show more than one error, and provide more meaningful error messages than the default messages.

peglint - PEG syntax lint utility

Build peglint

> cd lint
> mkdir build
> cd build
> cmake ..
> make
> ./peglint
usage: grammar_file_path [source_file_path]

  options:
    --source: source text
    --packrat: enable packrat memoise
    --ast: show AST tree
    --opt, --opt-all: optimize all AST nodes except nodes selected with `no_ast_opt` instruction
    --opt-only: optimize only AST nodes selected with `no_ast_opt` instruction
    --trace: show trace messages

Grammar check

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

> peglint a.peg
[commendline]:3:35: 'Number' is not defined.

Source check

> 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

AST

> 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)

AST optimazation

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

Adjust AST optimazation with no_ast_opt instruction

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

> peglint --ast --opt --source "1 + 2 * 3" a.peg
+ Additive/0
  + Multitive/1[Primary]
    - Number (1)
  + Additive/1[Multitive]
    + Primary/1
      - Number (2)
    + Multitive/1[Primary]
      - Number (3)

> peglint --ast --opt-only --source "1 + 2 * 3" a.peg
+ Additive/0
  + Multitive/1
    - Primary/1[Number] (1)
  + Additive/1
    + Multitive/0
      - Primary/1[Number] (2)
      + Multitive/1
        - Primary/1[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. :)

License

MIT license (© 2021 Yuji Hirose)