Here are some projects that use peg to provide further examples of PEG grammars:
- https://github.com/tj/go-naturaldate - natural date/time parsing
- https://github.com/gnames/gnparser - scientific names parsing
First declare the package name and any import(s) required.
package <package name>
import <import name>
Then declare the parser:
type <parser name> Peg {
<parser state variables>
}
Next declare the rules. Note that the main rules are described below but are based on the peg/leg rules which provide additional documentation.
The first rule is the entry point into the parser:
<rule name> <- <rule body>
The first rule should probably end with !. to indicate no more input follows:
first <- . !.
This is often set to END to make PEG rules more readable:
END <- !.
. means any character matches. For zero or more character matches, use:
repetition <- .*
For one or more character matches, use:
oneOrMore <- .+
For an optional character match, use:
optional <- .?
If specific characters are to be matched, use single quotes:
specific <- 'a'* 'bc'+ 'de'?
This will match the string "aaabcbcde".
For choosing between different inputs, use alternates:
prioritized <- 'a' 'a'* / 'bc'+ / 'de'?
This will match "aaaa" or "bcbc" or "de" or "". The matches are attempted in order.
If the characters are case-insensitive, use double quotes:
insensitive <- "abc"
This will match "abc" or "Abc" or "ABc" and so on.
For matching a set of characters, use a character class:
class <- [a-z]
This will match "a" or "b" or all the way to "z".
For an inverse character class, start with a caret:
inverse <- [^a-z]
This will match anything but "a" or "b" or all the way to "z".
If the character class is case-insensitive, use double brackets:
insensitive <- [[A-Z]]
(Note that this is not available in regular expression syntax.)
Use parentheses for grouping:
grouping <- (rule1 / rule2) rule3
For looking ahead a match (predicate), use:
lookAhead <- &rule1 rule2
For inverse look ahead, use:
inverse <- !rule1 rule2
Use curly braces for Go code:
gocode <- { fmt.Println("hello world") }
For string captures, use less than and greater than:
capture <- <'capture'> { fmt.Println(text) }
Will print out "capture". The captured string is stored in buffer[begin:end].
Use caution when picking your names to avoid overwriting existing .go files. Since only one PEG grammar is allowed per Go package (currently) the use of the name grammar.peg is suggested as a convention.
grammar.peg
grammar.go