*cpp-peglib* tries to provide more expressive parsing experience than common regular expression libraries such as std::regex. This library depends on only one header file. So, you can start using it right away just by including `peglib.h` in your project.
*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](http://pdos.csail.mit.edu/papers/parsing:popl04.pdf).
How to use
----------
What if we want to extract only tag names in brackets from ` [tag1] [tag2] [tag3] [tag4]... `? It's a bit hard to do it with *std::regex*, since it doesn't support [Repeated Captures](http://www.boost.org/doc/libs/1_57_0/libs/regex/doc/html/boost_regex/captures.html#boost_regex.captures.repeated_captures). PEG can, however, handle the repetition pretty easily.
What if we want to extract only tag names in brackets from ` [tag1] [tag2] [tag3] [tag4]... `?
PEG grammar for this task could be like this:
@ -49,9 +49,24 @@ assert(tags[1] == "tag:2");
assert(tags[2] == "tag-3");
```
You may have a question regarding '(3) Setup an action'. When the parser recognizes the definition 'TAG_NAME', it calls back the action `[&](const char* s, size_t l)` where `const char* s, size_t l` refers to the matched string, so that the user could use the string for something else.
This action `[&](const char* s, size_t l)` gives us the point and length of the matched string.
We can do more with actions.
There are more actions available. Here is a complete list:
`const std::vector<peglib::Any>& v` contains semantic values. `peglib::Any` class is very similar to [boost::any](http://www.boost.org/doc/libs/1_57_0/doc/html/any.html). You can obtain a value by castning it to the actual type. In order to determine the actual type, you have to check the return value type of the child action for the semantic value.
`const std::vector<std::string>& n` contains definition names of semantic values.
This is a complete code of a simple calculator. It shows how to associate actions to definitions and set/get semantic values.
auto ret = parser.parse("1+2*3*(4-5+6)/7-8", val);
assert(ret == true);
assert(val == -3);
```
It may be helpful to keep in mind that the action behavior is similar to the YACC semantic action model ($$, $1, $2, ...).
In this example, the actions return values. These samentic values will be pushed up to the parent definition which can be referred to in the parent action `[](const vector<Any>& v)`. In other words, when a certain definition has been accepted, we can find all semantic values which are associated with the child definitions in `const vector<Any>& v`. The values are wrapped by peglib::Any class which is like `boost::any`. We can retrieve the value by using `get<T>` method where `T` is the actual type of the value. If no value is returned in an action, an undefined `Any` will be pushed up to the parent. Finally, the resulting value of the root definition is received in the out parameter of `parse` method in the parser. `long val` is the resulting value in this case.
`const std::vector<std::string>& n` holds names of child definitions that could be helpful when we want to check what are the actual child definitions.
yhirose
9 years ago