#. First write a grammar. There are several ways to do that:
#. The canonical grammar format uses Python statements and expressions. Each rule is specified as Python function which should return a data structure that defines the rule. For example a grammar for simple calculator can be written as::
a) The canonical grammar format uses Python statements and expressions. Each rule is specified as Python function which should return a data structure that defines the rule. For example a grammar for simple calculator can be written as::
from arpeggio import Optional, ZeroOrMore, OneOrMore, EOF
from arpeggio import RegExMatch as _
...
...
@@ -53,23 +53,23 @@ Quick start
The python lists in the data structure represent ordered choices while the tuples represent sequences from the PEG.
For terminal matches use plain strings or regular expressions.
#. The same grammar could also be written using traditional textual PEG syntax like this::
b) The same grammar could also be written using traditional textual PEG syntax like this::
number <- r'\d*\.\d*|\d+'; // this is a comment
factor <- ("+" / "-")?
(number / "(" expression ")");
term <- factor (( "*" / "/") factor)*;
expression <- term (("+" / "-") term)*;
calc <- expression+ EOF;
number <- r'\d*\.\d*|\d+'; // this is a comment
factor <- ("+" / "-")?
(number / "(" expression ")");
term <- factor (( "*" / "/") factor)*;
expression <- term (("+" / "-") term)*;
calc <- expression+ EOF;
#. Or similar syntax but a little bit more readable like this::
c) Or similar syntax but a little bit more readable like this::
number = r'\d*\.\d*|\d+' # this is a comment
factor = ("+" / "-")?
(number / "(" expression ")")
term = factor (( "*" / "/") factor)*
expression = term (("+" / "-") term)*
calc = expression+ EOF
number = r'\d*\.\d*|\d+' # this is a comment
factor = ("+" / "-")?
(number / "(" expression ")")
term = factor (( "*" / "/") factor)*
expression = term (("+" / "-") term)*
calc = expression+ EOF
The second and third options are implemented using canonical first form. Feel free to implement your own grammar syntax if you don't like these (see modules :code:`arpeggio.peg` and :code:`arpeggio.cleanpeg`).
