Build Status

lexi

A regex-based lexer for dotnet. The lexer supports simple L1 recursive descent parsers.

Nuget Package

https://www.nuget.org/packages/MSL.Lexi/

dotnet add package MSL.Lexi

Sample Projects

I've included two sample projects in the repo to demonstrate the lexer within a recursive descent parser. One is a simple math parser and the other is a predicate expression parser. Each project includes a parser library, a set of tests for the parser, and a REPL console application that allows you to interact with the parser.

See Math.Parser and Predicate.Parser for working samples.

Sample Math.REPL Output

math:> (1 + 1) / 2 * 3
BinaryOperation
Left Expression
   BinaryOperation
   Left Expression
      Group Expression
         BinaryOperation
         Left Expression
            Number: 1
         Op Add
         Right Expression
            Number: 1
   Op Divide
   Right Expression
      Number: 2
Op Multiply
Right Expression
   Number: 3
-------------
result:> 3

math:>

Sample Predicate.REPL Output

predicate:> from Address where Street startswith "Cypress" and (City = "Tampa" or City = "Miami")
From: Address
LogicalExpression:
|-- L: ComparisonExpression:
|-- L: |-- L: Identifier: Street
|-- L: |-- Operator: StartsWith
|-- L: |-- R: StringLiteral: Cypress
|-- Operator: And
|-- R: ParentheticalExpression:
|-- R: |-- (: LogicalExpression:
|-- R: |-- (: |-- L: ComparisonExpression:
|-- R: |-- (: |-- L: |-- L: Identifier: City
|-- R: |-- (: |-- L: |-- Operator: Equal
|-- R: |-- (: |-- L: |-- R: StringLiteral: Tampa
|-- R: |-- (: |-- Operator: Or
|-- R: |-- (: |-- R: ComparisonExpression:
|-- R: |-- (: |-- R: |-- L: Identifier: City
|-- R: |-- (: |-- R: |-- Operator: Equal
|-- R: |-- (: |-- R: |-- R: StringLiteral: Miami
predicate:>

VocabularyBuilder Examples

You specify the vocabulary with the VocabularyBuilder which returns a lexer from the Build method.

Here's a sample from the Math.Parser project:

public static IServiceCollection AddParser(this IServiceCollection services)
{
    var builder = VocabularyBuilder
        .Create(RegexOptions.CultureInvariant)
        .Match("false", TokenIds.FALSE)
        .Match("true", TokenIds.TRUE)
        .Match(CommonPatterns.IntegerLiteral(), TokenIds.INTEGER_LITERAL)
        .Match(CommonPatterns.FloatingPointLiteral(), TokenIds.FLOATING_POINT_LITERAL)
        .Match(CommonPatterns.ScientificNotationLiteral(), TokenIds.SCIENTIFIC_NOTATION_LITERAL)
        .Match(@"\+", TokenIds.ADD)
        .Match("-", TokenIds.SUBTRACT)
        .Match(@"\*", TokenIds.MULTIPLY)
        .Match("/", TokenIds.DIVIDE)
        .Match("%", TokenIds.MODULUS)
        .Match(@"\(", TokenIds.OPEN_PARENTHESIS)
        .Match(@"\)", TokenIds.CLOSE_PARENTHESIS);

    // register the lexer with the service collection
    services.TryAddSingleton(serviceProvider => builder.Build());

    // lexer is injected into Parser constructor:
    // public sealed class Parser(Lexer lexer)
    services.TryAddTransient<Parser>();

    return services;
}

The Predicate.Parser project works the same way:

public static IServiceCollection AddParser(this IServiceCollection services)
{
    var builder = VocabularyBuilder
        .Create(RegexOptions.CultureInvariant | RegexOptions.IgnoreCase)
        .Match($"{nameof(TokenIds.FROM)}", TokenIds.FROM)
        .Match($"{nameof(TokenIds.WHERE)}", TokenIds.WHERE)
        .Match($"{nameof(TokenIds.SKIP)}", TokenIds.SKIP)
        .Match($"{nameof(TokenIds.TAKE)}", TokenIds.TAKE)
        .Match($"{nameof(TokenIds.CONTAINS)}", TokenIds.CONTAINS)
        .Match("startswith|sw", TokenIds.STARTS_WITH)
        .Match("endswith|ew", TokenIds.ENDS_WITH)
        .Match(@"and|&&", TokenIds.LOGICAL_AND)
        .Match(@"or|\|\|", TokenIds.LOGICAL_OR)
        .Match("null|NULL", TokenIds.NULL_LITERAL)
        .Match(CommonPatterns.Identifier(), TokenIds.IDENTIFIER)
        .Match("true", TokenIds.TRUE)
        .Match("false", TokenIds.FALSE)
        .Match(CommonPatterns.IntegerLiteral(), TokenIds.INTEGER_LITERAL)
        .Match(CommonPatterns.FloatingPointLiteral(), TokenIds.FLOATING_POINT_LITERAL)
        .Match(CommonPatterns.ScientificNotationLiteral(), TokenIds.SCIENTIFIC_NOTATION_LITERAL)
        .Match(CommonPatterns.QuotedStringLiteral(), TokenIds.STRING_LITERAL)
        .Match(CommonPatterns.CharacterLiteral(), TokenIds.CHAR_LITERAL)
        .Match(@"\(", TokenIds.OPEN_PARENTHESIS)
        .Match(@"\)", TokenIds.CLOSE_PARENTHESIS)
        .Match("=|==", TokenIds.EQUAL)
        .Match(">", TokenIds.GREATER_THAN)
        .Match(">=", TokenIds.GREATER_THAN_OR_EQUAL)
        .Match("<", TokenIds.LESS_THAN)
        .Match("<=", TokenIds.LESS_THAN_OR_EQUAL)
        .Match("!=", TokenIds.NOT_EQUAL);

    // register the lexer with the service collection
    services.TryAddSingleton(serviceProvider => builder.Build());

    // lexer is injected into Parser constructor:
    // public sealed class Parser(Lexer lexer)
    services.TryAddTransient<Parser>();

    return services;
}

Practical Parser Example

The Math.Parser implements a classic term/factor recursive descent parser. The parser returns an expression tree that can be evaluated to get the result. We use the lexer to get the next token with calls to one of the Lexer.NextMatch overloads as required.

using Lexi;
using Math.Parser.Exceptions;
using Math.Parser.Expressions;
using System.Diagnostics.CodeAnalysis;
using System.Globalization;

namespace Math.Parser;

public sealed class Parser(Lexer lexer)
{
    private readonly Lexer lexer = lexer
        ?? throw new ArgumentNullException(nameof(lexer));

    public Expression Parse(string source)
    {
        ArgumentNullException.ThrowIfNull(source);

        return ParseTerm(new Source(source))
            .Expression;
    }

    private readonly ref struct ParseResult(
        Expression expression,
        MatchResult matchResult)
    {
        public readonly Expression Expression = expression;
        public readonly MatchResult MatchResult = matchResult;
    }

    private ParseResult ParseTerm(Source script)
    {
        var left = ParseFactor(script);

        var matchResult = left.MatchResult;
        matchResult = lexer.NextMatch(matchResult);

        while (!matchResult.Source.IsEndOfSource
            && matchResult.Symbol.IsOperator()
            && matchResult.Symbol.IsTerm())
        {
            var right = ParseFactor(matchResult.Source);

            left = new(
                new BinaryOperation(
                left.Expression,
                right.Expression,
                matchResult.Symbol.TokenId),
                right.MatchResult);

            matchResult = lexer.NextMatch(right.MatchResult);
        }

        return left;
    }

    private ParseResult ParseFactor(Source script)
    {
        var left = ParseValue(script);

        var matchResult = left.MatchResult;
        matchResult = lexer.NextMatch(matchResult);

        while (!matchResult.Source.IsEndOfSource
            && matchResult.Symbol.IsOperator()
            && matchResult.Symbol.IsFactor())
        {
            var right = ParseValue(matchResult.Source);

            left = new(
                new BinaryOperation(
                left.Expression,
                right.Expression,
                matchResult.Symbol.TokenId),
                right.MatchResult);

            matchResult = lexer.NextMatch(right.MatchResult);
        }

        return left;
    }

    private ParseResult ParseValue(Source source)
    {
        if (source.IsEndOfSource)
        {
            throw new UnexpectedEndOfSourceException("Unexpected end of source");
        }

        var matchResult = lexer.NextMatch(source);

        if (matchResult.Symbol.IsNumericLiteral())
        {
            return new(ParseNumber(in matchResult), matchResult);
        }
        else if (matchResult.Symbol.IsOpenCircumfixDelimiter())
        {
            var term = ParseTerm(matchResult.Source);

            matchResult = lexer.NextMatch(term.MatchResult);
            if (matchResult.Symbol.IsCloseCircumfixDelimiter())
            {
                return new(new Group(term.Expression), matchResult);
            }

            if (matchResult.Symbol.IsMatch)
            {
                throw new UnexpectedTokenException($"unexpected token '{matchResult.Source.ReadSymbol(in matchResult.Symbol)}' at {matchResult.Symbol.Offset}. expected close parenthesis.");
            }

            throw new UnexpectedEndOfSourceException($"unexpected token '{source.Remaining()}' at {matchResult.Symbol.Offset}. expected close parenthesis.");
        }

        if (matchResult.Symbol.IsMatch)
        {
            throw new UnexpectedTokenException($"unexpected token '{matchResult.Source.ReadSymbol(in matchResult.Symbol)}' at {matchResult.Symbol.Offset}. expected number or open parenthesis.");
        }

        throw new UnexpectedTokenException($"unexpected token '{source.Remaining()}' at {matchResult.Symbol.Offset}. expected number or open parenthesis.");
    }

    [SuppressMessage("Style", "IDE0072:Add missing cases", Justification = "switch is complete")]
    private static Number ParseNumber(ref readonly MatchResult matchResult)
    {
        var value = matchResult
            .Source
            .ReadSymbol(in matchResult.Symbol);

        return matchResult.Symbol.TokenId switch
        {
            TokenIds.INTEGER_LITERAL => new Number(
                NumericTypes.Integer,
                Int32.Parse(value, NumberStyles.Integer, CultureInfo.InvariantCulture)),
            TokenIds.FLOATING_POINT_LITERAL => new Number(
                NumericTypes.FloatingPoint,
                Double.Parse(value, NumberStyles.Float, CultureInfo.InvariantCulture)),
            TokenIds.SCIENTIFIC_NOTATION_LITERAL => new Number(
                NumericTypes.ScientificNotation,
                Double.Parse(value, NumberStyles.Number | NumberStyles.AllowExponent, CultureInfo.InvariantCulture)),
            _ => new Number(NumericTypes.NotANumber, 0)
        };
    }
}