[](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, "~> 0.7.1"}
]
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 <- str(+{'a'..'z'})
Number <- int(+{'0'..'9'})
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 name is an elixir atom, in the form `:name` or `Name`,
whichever you prefer. A rule is made up of a number of _atoms_ (not to be
confused with Elixirs atoms. I should probably find another name for this) 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 `str()` 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 <- str(+{'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.
## Grammars
The `peg` macro provides a method to define (recursive) grammars. The first
argument is the name of initial patterns, followed by a list of named patterns.
Patterns can now refer to other patterns by name, allowing for recursion.
The order in which the grammar patterns are defined affects the generated
parser. Although Xpeg could always reorder, this is a design choice to give the
user more control over the generated parser:
- when a pattern P1 refers to pattern P2 which is defined before P1, P2 will
be inlined in P1. This increases the generated code size, but generally
improves performance.
- when a pattern P1 refers to pattern P2 which is defined after P1, P2 will be
generated as a subroutine which gets called from P1. This will reduce code
size, but might also result in a slower 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
@P # searches for P
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
```
## Performance
Generated parsers will typically never reach the spead of a hand-crafted and
fine tuned parser for a specific grammar. Having said that, Xpeg parsers can
still be pretty fast; for example, the JSON parser from the examples runs at
approximately 2/3 of the speed of the Poison JSON parser, which is said to be
"wicked-fast"
## TODO
- 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
