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597 lines
16 KiB
Markdown
597 lines
16 KiB
Markdown
cpp-peglib
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==========
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[](https://travis-ci.org/yhirose/cpp-peglib)
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[](https://ci.appveyor.com/project/yhirose/cpp-peglib)
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C++11 header-only [PEG](http://en.wikipedia.org/wiki/Parsing_expression_grammar) (Parsing Expression Grammars) library. You can start using it right away just by including `peglib.h` in your project.
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You can also try the online version, PEG Playground at https://yhirose.github.io/cpp-peglib.
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The PEG syntax is well described on page 2 in the [document](http://www.brynosaurus.com/pub/lang/peg.pdf). *cpp-peglib* also supports the following additional syntax for now:
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* `'...'i` (Case-insensitive literal operator)
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* `[^...]` (Negated character class operator)
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* `<` ... `>` (Token boundary operator)
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* `~` (Ignore operator)
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* `\x20` (Hex number char)
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* `%whitespace` (Automatic whitespace skipping)
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* `%word` (Word expression)
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* `$name(` ... `)` (Capture scope operator)
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* `$name<` ... `>` (Named capture operator)
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* `$name` (Backreference operator)
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* `MACRO_NAME(` ... `)` (Parameterized rule or Macro)
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This library also supports the linear-time parsing known as the [*Packrat*](http://pdos.csail.mit.edu/~baford/packrat/thesis/thesis.pdf) parsing.
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IMPORTANT NOTE for some Linux distributions such as Ubuntu and CentOS: Need `-pthread` option when linking. See [#23](https://github.com/yhirose/cpp-peglib/issues/23#issuecomment-261126127), [#46](https://github.com/yhirose/cpp-peglib/issues/46#issuecomment-417870473) and [#62](https://github.com/yhirose/cpp-peglib/issues/62#issuecomment-492032680).
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How to use
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----------
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This is a simple calculator sample. It shows how to define grammar, associate samantic actions to the grammar, and handle semantic values.
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```cpp
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// (1) Include the header file
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#include <peglib.h>
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#include <assert.h>
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#include <iostream>
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using namespace peg;
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using namespace std;
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int main(void) {
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// (2) Make a parser
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parser parser(R"(
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# Grammar for Calculator...
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Additive <- Multitive '+' Additive / Multitive
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Multitive <- Primary '*' Multitive / Primary
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Primary <- '(' Additive ')' / Number
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Number <- < [0-9]+ >
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%whitespace <- [ \t]*
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)");
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assert((bool)parser == true);
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// (3) Setup actions
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parser["Additive"] = [](const SemanticValues& sv) {
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switch (sv.choice()) {
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case 0: // "Multitive '+' Additive"
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return any_cast<int>(sv[0]) + any_cast<int>(sv[1]);
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default: // "Multitive"
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return any_cast<int>(sv[0]);
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}
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};
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parser["Multitive"] = [](const SemanticValues& sv) {
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switch (sv.choice()) {
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case 0: // "Primary '*' Multitive"
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return any_cast<int>(sv[0]) * any_cast<int>(sv[1]);
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default: // "Primary"
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return any_cast<int>(sv[0]);
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}
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};
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parser["Number"] = [](const SemanticValues& sv) {
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return stoi(sv.token(), nullptr, 10);
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};
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// (4) Parse
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parser.enable_packrat_parsing(); // Enable packrat parsing.
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int val;
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parser.parse(" (1 + 2) * 3 ", val);
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assert(val == 9);
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}
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```
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To show syntax errors in grammar text:
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```cpp
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auto grammar = R"(
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# Grammar for Calculator...
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Additive <- Multitive '+' Additive / Multitive
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Multitive <- Primary '*' Multitive / Primary
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Primary <- '(' Additive ')' / Number
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Number <- < [0-9]+ >
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%whitespace <- [ \t]*
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)";
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parser parser;
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parser.log = [](size_t line, size_t col, const string& msg) {
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cerr << line << ":" << col << ": " << msg << "\n";
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};
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auto ok = parser.load_grammar(grammar);
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assert(ok);
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```
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There are four semantic actions available:
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```cpp
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[](const SemanticValues& sv, any& dt)
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[](const SemanticValues& sv)
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[](SemanticValues& sv, any& dt)
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[](SemanticValues& sv)
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```
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`SemanticValues` value contains the following information:
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- Semantic values
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- Matched string information
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- Token information if the rule is literal or uses a token boundary operator
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- Choice number when the rule is 'prioritized choise'
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`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.
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`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`.
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To force using the simpler `any` implementation that comes with `cpp-peglib`, define `PEGLIB_USE_STD_ANY` as 0 before including `peglib.h`:
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```cpp
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#define PEGLIB_USE_STD_ANY 0
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#include <peglib.h>
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[...]
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```
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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.)
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Here shows the `SemanticValues` structure:
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```cpp
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struct SemanticValues : protected std::vector<any>
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{
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// Input text
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const char* path;
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const char* ss;
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// Matched string
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std::string str() const; // Matched string
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const char* c_str() const; // Matched string start
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size_t length() const; // Matched string length
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// Line number and column at which the matched string is
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std::pair<size_t, size_t> line_info() const;
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// Tokens
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std::vector<
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std::pair<
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const char*, // Token start
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size_t>> // Token length
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tokens;
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std::string token(size_t id = 0) const;
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// Choice number (0 based index)
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size_t choice() const;
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// Transform the semantic value vector to another vector
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template <typename T> vector<T> transform(size_t beg = 0, size_t end = -1) const;
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}
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```
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The following example uses `<` ... ` >` operator, which is *token boundary* operator.
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```cpp
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auto syntax = R"(
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ROOT <- _ TOKEN (',' _ TOKEN)*
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TOKEN <- < [a-z0-9]+ > _
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_ <- [ \t\r\n]*
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)";
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peg pg(syntax);
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pg["TOKEN"] = [](const SemanticValues& sv) {
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// 'token' doesn't include trailing whitespaces
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auto token = sv.token();
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};
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auto ret = pg.parse(" token1, token2 ");
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```
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We can ignore unnecessary semantic values from the list by using `~` operator.
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```cpp
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peg::parser parser(R"(
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ROOT <- _ ITEM (',' _ ITEM _)*
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ITEM <- ([a-z])+
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~_ <- [ \t]*
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)");
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parser["ROOT"] = [&](const SemanticValues& sv) {
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assert(sv.size() == 2); // should be 2 instead of 5.
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};
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auto ret = parser.parse(" item1, item2 ");
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```
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The following grammar is same as the above.
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```cpp
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peg::parser parser(R"(
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ROOT <- ~_ ITEM (',' ~_ ITEM ~_)*
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ITEM <- ([a-z])+
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_ <- [ \t]*
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)");
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```
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*Semantic predicate* support is available. We can do it by throwing a `peg::parse_error` exception in a semantic action.
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```cpp
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peg::parser parser("NUMBER <- [0-9]+");
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parser["NUMBER"] = [](const SemanticValues& sv) {
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auto val = stol(sv.str(), nullptr, 10);
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if (val != 100) {
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throw peg::parse_error("value error!!");
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}
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return val;
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};
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long val;
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auto ret = parser.parse("100", val);
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assert(ret == true);
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assert(val == 100);
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ret = parser.parse("200", val);
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assert(ret == false);
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```
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*enter* and *leave* actions are also avalable.
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```cpp
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parser["RULE"].enter = [](const char* s, size_t n, any& dt) {
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std::cout << "enter" << std::endl;
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};
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parser["RULE"] = [](const SemanticValues& sv, any& dt) {
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std::cout << "action!" << std::endl;
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};
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parser["RULE"].leave = [](const char* s, size_t n, size_t matchlen, any& value, any& dt) {
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std::cout << "leave" << std::endl;
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};
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```
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Ignoring Whitespaces
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--------------------
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As you can see in the first example, we can ignore whitespaces between tokens automatically with `%whitespace` rule.
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`%whitespace` rule can be applied to the following three conditions:
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* trailing spaces on tokens
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* leading spaces on text
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* trailing spaces on literal strings in rules
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These are valid tokens:
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```
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KEYWORD <- 'keyword'
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KEYWORDI <- 'case_insensitive_keyword'
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WORD <- < [a-zA-Z0-9] [a-zA-Z0-9-_]* > # token boundary operator is used.
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IDNET <- < IDENT_START_CHAR IDENT_CHAR* > # token boundary operator is used.
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```
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The following grammar accepts ` one, "two three", four `.
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```
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ROOT <- ITEM (',' ITEM)*
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ITEM <- WORD / PHRASE
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WORD <- < [a-z]+ >
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PHRASE <- < '"' (!'"' .)* '"' >
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%whitespace <- [ \t\r\n]*
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```
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Word expression
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---------------
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```cpp
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peg::parser parser(R"(
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ROOT <- 'hello' 'world'
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%whitespace <- [ \t\r\n]*
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%word <- [a-z]+
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)");
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parser.parse("hello world"); // OK
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parser.parse("helloworld"); // NG
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```
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Capture/Backreference
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---------------------
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```cpp
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peg::parser parser(R"(
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ROOT <- CONTENT
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CONTENT <- (ELEMENT / TEXT)*
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ELEMENT <- $(STAG CONTENT ETAG)
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STAG <- '<' $tag< TAG_NAME > '>'
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ETAG <- '</' $tag '>'
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TAG_NAME <- 'b' / 'u'
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TEXT <- TEXT_DATA
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TEXT_DATA <- ![<] .
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)");
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parser.parse("This is <b>a <u>test</u> text</b>."); // OK
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parser.parse("This is <b>a <u>test</b> text</u>."); // NG
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parser.parse("This is <b>a <u>test text</b>."); // NG
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```
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Parameterized Rule or Macro
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---------------------------
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```peg
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# Syntax
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Start ← _ Expr
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Expr ← Sum
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Sum ← List(Product, SumOpe)
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Product ← List(Value, ProOpe)
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Value ← Number / T('(') Expr T(')')
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# Token
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SumOpe ← T('+' / '-')
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ProOpe ← T('*' / '/')
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Number ← T([0-9]+)
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~_ ← [ \t\r\n]*
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# Macro
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List(I, D) ← I (D I)*
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T(x) ← < x > _
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```
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Parsing expressions by precedence climbing altorithm
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----------------------------------------------------
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*cpp-peglib* supports [operator-precedence parsering](https://en.wikipedia.org/wiki/Operator-precedence_parser) by [**precedence climbing algorithm**](https://eli.thegreenplace.net/2012/08/02/parsing-expressions-by-precedence-climbing)
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```cpp
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parser parser(R"(
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EXPRESSION <- ATOM (OPERATOR ATOM)* {
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precedence
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L - +
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L / *
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}
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ATOM <- NUMBER / '(' EXPRESSION ')'
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OPERATOR <- < [-+/*] >
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NUMBER <- < '-'? [0-9]+ >
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%whitespace <- [ \t\r\n]*
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)");
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parser["EXPRESSION"] = [](const SemanticValues& sv) -> long {
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auto result = any_cast<long>(sv[0]);
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if (sv.size() > 1) {
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auto ope = any_cast<char>(sv[1]);
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auto num = any_cast<long>(sv[2]);
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switch (ope) {
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case '+': result += num; break;
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case '-': result -= num; break;
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case '*': result *= num; break;
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case '/': result /= num; break;
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}
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}
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return result;
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};
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parser["OPERATOR"] = [](const SemanticValues& sv) { return *sv.c_str(); };
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parser["NUMBER"] = [](const SemanticValues& sv) { return atol(sv.c_str()); };
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long val;
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parser.parse(" -1 + (1 + 2) * 3 - -1", val);
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assert(val == 9);
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```
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AST generation
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--------------
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*cpp-peglib* is able to generate an AST (Abstract Syntax Tree) when parsing. `enable_ast` method on `peg::parser` class enables the feature.
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```
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peg::parser parser("...");
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parser.enable_ast();
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shared_ptr<peg::Ast> ast;
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if (parser.parse("...", ast)) {
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cout << peg::ast_to_s(ast);
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ast = peg::AstOptimizer(true).optimize(ast);
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cout << peg::ast_to_s(ast);
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}
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```
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`peg::AstOptimizer` removes redundant nodes to make a AST simpler. You can make your own AST optimizers to fit your needs.
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See actual usages in the [AST calculator example](https://github.com/yhirose/cpp-peglib/blob/master/example/calc3.cc) and [PL/0 language example](https://github.com/yhirose/cpp-peglib/blob/master/pl0/pl0.cc).
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Make a parser with parser combinators
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-------------------------------------
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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:
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```cpp
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using namespace peg;
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using namespace std;
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vector<string> tags;
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Definition ROOT, TAG_NAME, _;
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ROOT <= seq(_, zom(seq(chr('['), TAG_NAME, chr(']'), _)));
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TAG_NAME <= oom(seq(npd(chr(']')), dot())), [&](const SemanticValues& sv) {
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tags.push_back(sv.str());
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};
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_ <= zom(cls(" \t"));
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auto ret = ROOT.parse(" [tag1] [tag:2] [tag-3] ");
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```
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The following are available operators:
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| Operator | Description |
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| :------- | :------------------------------ |
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| seq | Sequence |
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| cho | Prioritized Choice |
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| zom | Zero or More |
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| oom | One or More |
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| opt | Optional |
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| apd | And predicate |
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| npd | Not predicate |
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| lit | Literal string |
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| liti | Case-insensitive Literal string |
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| cls | Character class |
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| ncls | Negated Character class |
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| chr | Character |
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| dot | Any character |
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| tok | Token boundary |
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| ign | Ignore semantic value |
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| csc | Capture scope |
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| cap | Capture |
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| bkr | Back reference |
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| usr | User defined parser |
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Adjust definitions
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------------------
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It's possible to add/override definitions.
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```cpp
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auto syntax = R"(
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ROOT <- _ 'Hello' _ NAME '!' _
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)";
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Rules additional_rules = {
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{
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"NAME", usr([](const char* s, size_t n, SemanticValues& sv, any& dt) -> size_t {
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static vector<string> names = { "PEG", "BNF" };
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for (const auto& name: names) {
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if (name.size() <= n && !name.compare(0, name.size(), s, name.size())) {
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return name.size(); // processed length
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}
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}
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return -1; // parse error
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})
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},
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{
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"~_", zom(cls(" \t\r\n"))
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}
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};
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auto g = parser(syntax, additional_rules);
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assert(g.parse(" Hello BNF! "));
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```
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Unicode support
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---------------
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cpp-peglib accepts UTF8 text. `.` matches a Unicode codepoint. Also, it supports `\u????`.
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peglint - PEG syntax lint utility
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---------------------------------
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### Build peglint
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```
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> cd lint
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> mkdir build
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> cd build
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> cmake ..
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> make
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> ./peglint
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usage: peglint [--ast] [--optimize_ast_nodes|--opt] [--source text] [--trace] [grammar file path] [source file path]
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```
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### Lint grammar
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```
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> cat a.peg
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A <- 'hello' ^ 'world'
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> peglint a.peg
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a.peg:1:14: syntax error
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```
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```
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> cat a.peg
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A <- B
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> peglint a.peg
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a.peg:1:6: 'B' is not defined.
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```
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```
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> cat a.peg
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A <- B / C
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B <- 'b'
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C <- A
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> peglint a.peg
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a.peg:1:10: 'C' is left recursive.
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a.peg:3:6: 'A' is left recursive.
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```
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### Lint source text
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```
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> cat a.peg
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Additive <- Multitive '+' Additive / Multitive
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Multitive <- Primary '*' Multitive / Primary
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Primary <- '(' Additive ')' / Number
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Number <- < [0-9]+ >
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%whitespace <- [ \t\r\n]*
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|
> peglint --source "1 + a * 3" a.peg
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|
[commendline]:1:3: syntax error
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|
```
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|
|
|
```
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|
> cat a.txt
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|
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](https://github.com/yhirose/cpp-peglib/blob/master/example/calc.cc)
|
|
* [Calculator (with parser operators)](https://github.com/yhirose/cpp-peglib/blob/master/example/calc2.cc)
|
|
* [Calculator (AST version)](https://github.com/yhirose/cpp-peglib/blob/master/example/calc3.cc)
|
|
* [PL/0 language example](https://github.com/yhirose/cpp-peglib/blob/master/pl0/pl0.cc)
|
|
* [A tiny PL/0 JIT compiler in less than 700 LOC with LLVM and PEG parser](https://github.com/yhirose/pl0-jit-compiler)
|
|
|
|
PEG debug
|
|
---------
|
|
|
|
A debug viewer for Parsing Expression Grammars using cpp-peglib by [mqnc](https://github.com/mqnc). Please see [his gihub project page](https://github.com/mqnc/pegdebug) for the detail. You can see a parse result of PL/0 code [here](https://mqnc.github.io/pegdebug/example/output.html).
|
|
|
|
TODO
|
|
----
|
|
|
|
* Advanced Unicode support ([Unicode regular expressoin](http://www.unicode.org/reports/tr18/))
|
|
|
|
License
|
|
-------
|
|
|
|
MIT license (© 2020 Yuji Hirose)
|