repeg

repeg is a java library to for creating parsers based on the principals of Parsing Expression Grammars (PEG).

This project was created back in 2010 but was cleaned up and published in 2019. Since it has only just been made public, consider this project in beta phase.

Work is ongoing to get this library published to maven central. First full release ETA end Q1-2020

Quick Links

Requirements

Java 1.8 or higher.

Main features

Parsers/Expressions are defined in code (java) using fluent API. Ease of use and understanding are primary objectives of this project.
Zero transitive dependencies - repeg does not require and further libraries
Plain old java (i.e. no byte code magic) so easy to integrate in restricted environments.

Future plans

Ability to define parsers in a dedicated DSL
Generate standalone parser with no dependencies to repeg
Built-in support for indented languages (such as python or yaml)
Investigate ratpack style parsing (Tests so far do not suggest much need for this. I would welcome any feedback and examples of languages where ratpack parsing may be of an advantage).

Examples

This project comes with several examples which you can use to get you started:

Calculator - parse and evaluate simple maths expressions such as (7+2)*5.
Template - a very simple template language mimicking the style of jtwig.
Todo - a parser for a text based todo list.
IsoDuration - a (partial) implementation of a parser for ISO durations such as P1Y6MT7M3S.
Persons (CSV) - an parser for CSV file containing person information.
Persons (JSON) - same as above, but parsing JSON input.

Note: The last 3 examples are provided to show what can be done but not necessarily what should be done with repeg. A parser for an ISO Duration can be hand-made relatively easily and will deliver much better performance. For common formats such as CSV and JSON, there are plenty of specialised parsers/libraries available which will handle corner-cases better than the examples and perform much better at the same time.

Test

public class ParserFactory extends ParserFactoryBase<IsoDuration> {

	@Override
	protected RuleEnum getStartRule() {
		return Rules.START;
	}

	public enum Rules implements RuleEnum {
		START {
			@Override
			public ExpressionBuilder expression() {
				return sequence( //
						one('P'), //
						Period_Part, //
						Time_Part, //
						endOfInput() //
				).onSuccess(context -> {
					Period period = context.get(0);
					Duration duration = context.get(1);
					context.replaceWith(IsoDuration.of(period, duration));
				});
			}
		},

		Period_Part {
			@Override
			public ExpressionBuilder expression() {
				return sequence( //
						// TODO: not 'T'
						optional(sequence(Number, one('Y'))).onSuccess(pushIfEmpty(0)), //
						optional(sequence(Number, one('M'))).onSuccess(pushIfEmpty(0)), //
						optional(sequence(Number, one('D'))).onSuccess(pushIfEmpty(0)) //
				).onSuccess((context) -> {
					int years = context.get(0);
					int months = context.get(1);
					int days = context.get(2);
					context.replaceWith(Period.of(years, months, days));
				});
			}
		},

		Time_Part {
			@Override
			public ExpressionBuilder expression() {
				return optional( //
						sequence( //
								one('T'), //
								optional(sequence(Number, one('H'))).onSuccess(pushIfEmpty(0)), //
								optional(sequence(Number, one('M'))).onSuccess(pushIfEmpty(0)), //
								optional(sequence(Number, one('S'))).onSuccess(pushIfEmpty(0)) //
						).onSuccess(context -> {
							int hours = context.get(0);
							int minutes = context.get(1);
							int seconds = context.get(2);

							int totalSeconds = (((hours * 60) + minutes) * 60) + seconds;
							context.replaceWith(Duration.ofSeconds(totalSeconds));
						}) //
				).onSuccess(pushIfEmpty(Duration.ofSeconds(0)));
			}
		},

		Number {
			@Override
			public ExpressionBuilder expression() {
				return oneOrMore(characterInRange('0', '9')).onSuccess(pushTextAsInteger());
			}
		};
	}
}