PEG file syntax

May 21, 2026 · View on GitHub

Examples of PEG grammars

Here are some projects that use peg to provide further examples of PEG grammars:

First declare the package name and any import(s) required.

package <package name>

import "<import path>"
import <alias> "<import path>"

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].