[](https://opensource.org/licenses/MIT)
More documentation will be added, for now please refer to the documentation of
[NPeg](https://github.com/zevv/npeg), a Nim implementation of a similar PEG
parser.
## Introduction
XPeg is a pure Elixir pattern matching library. It provides macros to compile
patterns and grammars (PEGs) to Elixir function which will parse a string and
collect selected parts of the input. PEGs are not unlike regular expressions,
but offer more power and flexibility, and have less ambiguities. (More about
PEGs on [Wikipedia](https://en.wikipedia.org/wiki/Parsing_expression_grammar))
Some use cases where XPeg is useful are configuration or data file parsers,
robust protocol implementations, input validation, lexing of programming
languages or domain specific languages.
## Installation
```elixir
def deps do
[
{:xpeg, "~> 0.4.0"}
]
end
```
## Quickstart
Here is a simple example showing the power of XPeg: The macro `peg` compiles a
grammar definition into a `parser` functiion, which is used to match a string and
place the key-value pairs into a list of tuples:
```elixir
p = Xpeg.peg :dict do
:dict <- :pair * star("," * :pair) * !1
:pair <- :word * "=" * :number * fn [a,b|cs] -> [{b,a}|cs] end
:word <- cap(+{'a'..'z'})
:number <- cap(+{'0'..'9'}) * fn [v|cs] -> [String.to_integer(v) | cs] end
end
Xpeg.match(p, "grass=4,horse=1,star=2")
```
Output:
```elixir
[{"star", 2}, {"horse", 1}, {"grass", 4}]
```
## Usage
The basic operation consists of the provided _grammar_, which consists of a set
of named _rules_. A rule is made up of a number of _atoms_ (not to be confused
with Elixirs atom) and _operators_, which are executed to match the input
string. Rules can also call into other rules, allowing for recursive grammars.
For example, the grammar below matches a comma-separated list of words
```elixir
p = peg :list do
:list <- :word * star( "," * :word )
:word <- +{'a'..'z'}
end
```
- The `:list` rule matches one `:word`, followed by zero or more (`star(P)`)
times a `,` followed by a `:word`
- The `:word` rule matches one-or-more (`+P`) times the set of characters (`{}`)
consisting of all letters from `'a'` to `'z'`
During the execution of the grammar, matching parts of the subject strings can
be _captured_ with the `cap()` operator. All captures are stored on the
`captures` list inside the parser state. This list is returned by the `match()`
function, but can also be used by in-grammar functions to perform conversions
or transformations.
Below is the same grammar as above, but in this case it captures all
the individual `:word:`s:
```elixir
p = peg :list do
:list <- :word * star( "," * :word )
:word <- cap(+{'a'..'z'})
end
match(p, "one,two,three")
```
The above will return these following list of captures:
```elixir
["three", "two", "one"]
```
A powerful feature allows mixing of Elixir functions with the grammar, which
can be used to perform transformations of the captures or build abstract syntax
trees (ASTs) on-the-fly.
For example, the grammar above is changed to match numbers instead of words,
and a conversion function is called after every matching number that
converts the last captured value on the `captures` list to an integer:
```elixir
p = peg :list do
:list <- :word * star( "," * :word )
:word <- cap(+{'0'..'9'}) *
fn [v|cs] ->
[String.to_integer(v)|cs]
end
end
match(p, "123,42,31415")
```
which results in the following captures:
```elixir
[31415, 42, 123]
```
More elaborate examples can be found in [examples_test.exs](/test/examples_test.exs),
including a parser for arithmatic expressions and a full JSON parser.
## Syntax
The XPeg syntax is similar to normal PEG notation, but some changes were made
to allow the grammar to be properly parsed by the Elixir compiler:
- XPeg uses prefix operators instead of suffix operators for `+`, `-`
- Elixir does not support the `*` and `?` prefix operators, so instead
`star(P)` and `opt(P)` are used
- The explicit `*` infix operator is used for concatenation
XPeg patterns and grammars can be composed of the following parts:
```
Atoms:
0 # matches always and consumes nothing
1 # matches any character
n # matches exactly n characters
'x' # matches literal character 'x'
"xyz" # matches literal string "xyz"
{'x'..'y'} # matches any character in the range from 'x'..'y'
{'x','y','z'} # matches any character from the set
Operators:
P1 * P2 # concatenation
P1 | P2 # ordered choice
P1 - P2 # matches P1 if P2 does not match
(P) # grouping
!P # matches everything but P
&P # matches P without consuming input
opt(P) # matches P zero or one times
star(P) # matches P zero or more times
+P # matches P one or more times
P[n] # matches P n times
P[m..n] # matches P m to n times
Captures:
str(P) # Adds the matched string to the capture list
int(P) # Adds the matched integer to the capture list
float(P) # Adds the matched float to the capture list
Elixir function:
fn(captures) # Elixir function for transormations
```
## TODO
XPeg is already useful, but still young and immature. Here's a list of things I want
to fix:
- I do not like the `star()` and `opt()` syntax of the AST, but given the limited
support for prefix operators in Elixir I'm not yet sure how to make this better
