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README


                                  ~* Citrus *~

                          Parsing Expressions for Ruby


Citrus is a compact and powerful parsing library for 
[Ruby](http://ruby-lang.org/) that combines the elegance and expressiveness of 
the language with the simplicity and power of
[parsing expressions](http://en.wikipedia.org/wiki/Parsing_expression_grammar).


# Installation


Via [RubyGems](http://rubygems.org/):

    $ sudo gem install citrus

From a local copy:

    $ git clone git://github.com/mjijackson/citrus.git
    $ cd citrus
    $ rake package && sudo rake install


# Background


In order to be able to use Citrus effectively, you must first understand the
difference between syntax and semantics. Syntax is a set of rules that govern
the way letters and punctuation may be used in a language. For example, English
syntax dictates that proper nouns should start with a capital letter and that
sentences should end with a period.

Semantics are the rules by which meaning may be derived in a language. For
example, as you read a book you are able to make some sense of the particular
way in which words on a page are combined to form thoughts and express ideas
because you understand what the words themselves mean and you understand what
they mean collectively.

Computers use a similar process when interpreting code. First, the code must be
parsed into recognizable symbols or tokens. These tokens may then be passed to
an interpreter which is responsible for forming actual instructions from them.

Citrus is a pure Ruby library that allows you to perform both lexical analysis
and semantic interpretation quickly and easily. Using Citrus you can write
powerful parsers that are simple to understand and easy to create and maintain.

In Citrus, there are three main types of objects: rules, grammars, and matches.

## Rules

A [Rule](api/classes/Citrus/Rule.html) is an object that specifies some matching
behavior on a string. There are two types of rules: terminals and non-terminals.
Terminals can be either Ruby strings or regular expressions that specify some
input to match. For example, a terminal created from the string "end" would
match any sequence of the characters "e", "n", and "d", in that order. A
terminal created from a regular expression uses Ruby's regular expression engine
to attempt to create a match.

Non-terminals are rules that may contain other rules but do not themselves match
directly on the input. For example, a Repeat is a non-terminal that may contain
one other rule that will try and match a certain number of times. Several other
types of non-terminals are available that will be discussed later.

Rule objects may also have semantic information associated with them in the form
of Ruby modules. Rules use these modules to extend the matches they create.

## Grammars

A grammar is a container for rules. Usually the rules in a grammar collectively
form a complete specification for some language, or a well-defined subset
thereof.

A Citrus grammar is really just a souped-up Ruby
[module](http://ruby-doc.org/core/classes/Module.html). These modules may be
included in other grammar modules in the same way that Ruby modules are normally
used. This property allows you to divide a complex grammar into more manageable, 
reusable pieces that may be combined at runtime. Any grammar rule with the same 
name as a rule in an included grammar may access that rule with a mechanism 
similar to Ruby's super keyword.

## Matches

Matches are created by rule objects when they match on the input. A 
[Match](api/classes/Citrus/Match.html) in Citrus is actually a 
[String](http://ruby-doc.org/core/classes/String.html) with some extra 
information attached such as the name(s) of the rule(s) which generated the 
match as well as its offset in the original input string.

During a parse, matches are arranged in a tree structure where any match may
contain any number of other matches. This structure is determined by the way in
which the rule that generated each match is used in the grammar. For example, a 
match that is created from a non-terminal rule that contains several other 
terminals will likewise contain several matches, one for each terminal.

Match objects may be extended with semantic information in the form of methods.
These methods can interpret the text of a match using the wealth of information
available to them including the text of the match, its position in the input,
and any submatches.


# Syntax


The most straightforward way to compose a Citrus grammar is to use Citrus' own
custom grammar syntax. This syntax borrows heavily from Ruby, so it should
already be familiar to Ruby programmers.

## Terminals

Terminals may be represented by a string or a regular expression. Both follow
the same rules as Ruby string and regular expression literals.

    'abc'
    "abc\n"
    /\xFF/

Character classes and the dot (match anything) symbol are supported as well for
compatibility with other parsing expression implementations.

    [a-z0-9]      # match any lowercase letter or digit
    [\x00-\xFF]   # match any octet
    .             # match anything, even new lines

See [FixedWidth](api/classes/Citrus/FixedWidth.html) and
[Expression](api/classes/Citrus/Expression.html) for more information.

## Repetition

Quantifiers may be used after any expression to specify a number of times it
must match. The universal form of a quantifier is N*M where N is the minimum and
M is the maximum number of times the expression may match.

    'abc'1*2      # match "abc" a minimum of one, maximum
                  # of two times
    'abc'1*       # match "abc" at least once
    'abc'*2       # match "abc" a maximum of twice

The + and ? operators are supported as well for the common cases of 1* and *1
respectively.

    'abc'+        # match "abc" at least once
    'abc'?        # match "abc" a maximum of once

See [Repeat](api/classes/Citrus/Repeat.html) for more information.

## Lookahead

Both positive and negative lookahead are supported in Citrus. Use the & and !
operators to indicate that an expression either should or should not match. In
neither case is any input consumed.

    &'a' 'b'      # match a "b" preceded by an "a"
    !'a' 'b'      # match a "b" that is not preceded by an "a"
    !'a' .        # match any character except for "a"

A special form of lookahead is also supported which will match any character
that does not match a given expression.

    ~'a'          # match all characters until an "a"
    ~/xyz/        # match all characters until /xyz/ matches

See [AndPredicate](api/classes/Citrus/AndPredicate.html),
[NotPredicate](api/classes/Citrus/NotPredicate.html), and
[ButPredicate](api/classes/Citrus/ButPredicate.html) for more information.

## Sequences

Sequences of expressions may be separated by a space to indicate that the rules
should match in that order.

    'a' 'b' 'c'   # match "a", then "b", then "c"
    'a' [0-9]     # match "a", then a numeric digit

See [Sequence](api/classes/Citrus/Sequence.html) for more information.

## Choices

Ordered choice is indicated by a vertical bar that separates two expressions.
Note that any operator binds more tightly than the bar.

    'a' | 'b'       # match "a" or "b"
    'a' 'b' | 'c'   # match "a" then "b" (in sequence), or "c"

See [Choice](api/classes/Citrus/Choice.html) for more information.

## Super

When including a grammar inside another, all rules in the child that have the
same name as a rule in the parent also have access to the "super" keyword to
invoke the parent rule.

See [Super](api/classes/Citrus/Super.html) for more information.

## Labels

Match objects may be referred to by a different name than the rule that
originally generated them. Labels are created by placing the label and a colon
immediately preceding any expression.

    chars:/[a-z]+/  # the characters matched by the regular
                    # expression may be referred to as "chars"
                    # in a block method

See [Label](api/classes/Citrus/Label.html) for more information.

## Precedence

The following table contains a list of all Citrus operators and their 
precedence. A higher precedence indicates tighter binding.

| Operator    | Name                      | Precedence |
| ----------- | ------------------------- | ---------- |
| ''          | String (single quoted)    | 6          |
| ""          | String (double quoted)    | 6          |
| []          | Character class           | 6          |
| .           | Dot (any character)       | 6          |
| //          | Regular expression        | 6          |
| ()          | Grouping                  | 6          |
| *           | Repetition (arbitrary)    | 5          |
| +           | Repetition (one or more)  | 5          |
| ?           | Repetition (zero or one)  | 5          |
| &           | And predicate             | 4          |
| !           | Not predicate             | 4          |
| ~           | But predicate             | 4          |
| :           | Label                     | 4          |
| <>          | Extension (module name)   | 3          |
| {}          | Extension (literal)       | 3          |
| e1 e2       | Sequence                  | 2          |
| e1 | e2     | Ordered choice            | 1          |


# Example


Below is an example of a simple grammar that is able to parse strings of
integers separated by any amount of white space and a `+` symbol.

    grammar Addition
      rule additive
        number plus (additive | number)
      end

      rule number
        [0-9]+ space
      end

      rule plus
        '+' space
      end

      rule space
        [ \t]*
      end
    end

Several things to note about the above example:

* Grammar and rule declarations end with the `end` keyword
* A Sequence of rules is created by separating expressions with a space
* Likewise, ordered choice is represented with a vertical bar
* Parentheses may be used to override the natural binding order
* Rules may refer to other rules in their own definitions simply by using the
  other rule's name
* Any expression may be followed by a quantifier

## Interpretation

The grammar above is able to parse simple mathematical expressions such as "1+2"
and "1 + 2+3", but it does not have enough semantic information to be able to
actually interpret these expressions.

At this point, when the grammar parses a string it generates a tree of
[Match](api/classes/Citrus/Match.html) objects. Each match is created by a rule.
A match knows what text it contains, its offset in the original input, and what
submatches it contains.

Submatches are created whenever a rule contains another rule. For example, in
the grammar above the number rule matches a string of digits followed by white
space. Thus, a match generated by the number rule will contain two submatches.

We can use Ruby's block syntax to create a module that will be attached to these
matches when they are created and is used to lazily extend them when we want to
interpret them. The following example shows one way to do this.

    grammar Addition
      rule additive
        (number plus term:(additive | number)) {
          def value
            number.value + term.value
          end
        }
      end

      rule number
        ([0-9]+ space) {
          def value
            strip.to_i
          end
        }
      end

      rule plus
        '+' space
      end

      rule space
        [ \t]*
      end
    end

In this version of the grammar we have added two semantic blocks, one each for
the additive and number rules. These blocks contain methods that will be present
on all match objects that result from matches of those particular rules. It's
easiest to explain what is going on here by starting with the lowest level
block, which is defined within the number rule.

The semantic block associated with the number rule defines one method, value.
Inside this method, we can see that the value of a number match is determined to
be its text value, stripped of white space and converted to an integer. Remember
that matches are simply strings, so the `strip` method in this case is actually
`String#strip`.

The `additive` rule also extends its matches with a value method. Notice the use
of the `term` label within the rule definition. This label allows the match that
is created by either the additive or the number rule to be retrieved using the
`term` label. The value of an additive is determined to be the values of its
`number` and `term` matches added together using Ruby's addition operator.

Since additive is the first rule defined in the grammar, any match that results
from parsing a string with this grammar will have a `value` method that can be
used to recursively calculate the collective value of the entire match tree.

To give it a try, save the code for the Addition grammar in a file called
addition.citrus. Next, assuming you have the Citrus gem installed, try the
following sequence of commands in a terminal.

    $ irb
    > require 'citrus'
     => true
    > Citrus.load 'addition'
     => [Addition]
    > m = Addition.parse '1 + 2 + 3'
     => #<Citrus::Match ...
    > m.value
     => 6

Congratulations! You just ran your first piece of Citrus code.

Take a look at 
[examples/calc.citrus](http://github.com/mjijackson/citrus/blob/master/examples/calc.citrus)
for an example of a calculator that is able to parse and evaluate more complex 
mathematical expressions.

## Implicit Value

It is very common for a grammar to only have one interpretation for a given
symbol. For this reason, you may find yourself writing a `value` method for
every rule in your grammar. Because this can be tedious, Citrus allows you to
omit defining such a method if you choose. For example, the `additive` and
`number` rules from the simple calculator example above could also be written
as:

    rule additive
      (number plus term:(additive | number)) {
        number.value + term.value
      }
    end

    rule number
      ([0-9]+ space) {
        strip.to_i
      }
    end

Since no method name is explicitly specified in the semantic blocks, they may be 
called using the `value` method.


# Links


The primary resource for all things to do with parsing expressions can be found
on the original [Packrat and Parsing Expression Grammars page](http://pdos.csail.mit.edu/~baford/packrat) at MIT.

Also, a useful summary of parsing expression grammars can be found on 
[Wikipedia](http://en.wikipedia.org/wiki/Parsing_expression_grammar).

Citrus draws inspiration from another Ruby library for writing parsing
expression grammars, Treetop. While Citrus' syntax is similar to that of
[Treetop](http://treetop.rubyforge.org), it's not identical. The link is 
included here for those who may wish toexplore an alternative implementation.


# License


Copyright 2010 Michael Jackson

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.
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