我正在尝试用 C# 编写一个非常简单的解析器。

我需要一个词法分析器——它可以让我将正则表达式与标记相关联，因此它会读取正则表达式并返回符号。

似乎我应该能够使用 Regex 来完成实际的繁重工作，但我看不到一种简单的方法来完成它。一方面，正则表达式似乎只适用于字符串，而不适用于流(这是为什么!？!？)。

基本上，我想要以下接口(interface)的实现:

interface ILexer : IDisposable
{
    /// <summary>
    /// Return true if there are more tokens to read
    /// </summary>
    bool HasMoreTokens { get; }
    /// <summary>
    /// The actual contents that matched the token
    /// </summary>
    string TokenContents { get; }
    /// <summary>
    /// The particular token in "tokenDefinitions" that was matched (e.g. "STRING", "NUMBER", "OPEN PARENS", "CLOSE PARENS"
    /// </summary>
    object Token { get; }
    /// <summary>
    /// Move to the next token
    /// </summary>
    void Next();
}

interface ILexerFactory
{
    /// <summary>
    /// Create a Lexer for converting a stream of characters into tokens
    /// </summary>
    /// <param name="reader">TextReader that supplies the underlying stream</param>
    /// <param name="tokenDefinitions">A dictionary from regular expressions to their "token identifers"</param>
    /// <returns>The lexer</returns>
    ILexer CreateLexer(TextReader reader, IDictionary<string, object> tokenDefinitions);
}

所以，请发送鳕鱼...
不，说真的，我正要开始编写上述接口(interface)的实现，但我很难相信在 .NET (2.0) 中已经没有一些简单的方法可以做到这一点。

那么，对于执行上述操作的简单方法有什么建议吗？ (此外，我不想要任何“代码生成器”。性能对于这件事并不重要，我不想在构建过程中引入任何复杂性。)

最佳答案

我在这里作为答案发布的原始版本有一个问题，因为它只有在有多个“正则表达式”匹配当前表达式时才有效。也就是说，一旦只有一个 Regex 匹配，它就会返回一个标记——而大多数人希望 Regex 是“贪婪的”。对于“带引号的字符串”之类的情况尤其如此。

位于 Regex 之上的唯一解决方案是逐行读取输入(这意味着您不能拥有跨越多行的标记)。我可以忍受这个——毕竟，这是一个穷人的词法分析器!此外，在任何情况下，从 Lexer 中获取行号信息通常都是有用的。

因此，这是一个解决这些问题的新版本。也归功于 this

public interface IMatcher
{
    /// <summary>
    /// Return the number of characters that this "regex" or equivalent
    /// matches.
    /// </summary>
    /// <param name="text">The text to be matched</param>
    /// <returns>The number of characters that matched</returns>
    int Match(string text);
}

sealed class RegexMatcher : IMatcher
{
    private readonly Regex regex;
    public RegexMatcher(string regex) => this.regex = new Regex(string.Format("^{0}", regex));

    public int Match(string text)
    {
        var m = regex.Match(text);
        return m.Success ? m.Length : 0;
    }
    public override string ToString() => regex.ToString();
}

public sealed class TokenDefinition
{
    public readonly IMatcher Matcher;
    public readonly object Token;

    public TokenDefinition(string regex, object token)
    {
        this.Matcher = new RegexMatcher(regex);
        this.Token = token;
    }
}

public sealed class Lexer : IDisposable
{
    private readonly TextReader reader;
    private readonly TokenDefinition[] tokenDefinitions;

    private string lineRemaining;

    public Lexer(TextReader reader, TokenDefinition[] tokenDefinitions)
    {
        this.reader = reader;
        this.tokenDefinitions = tokenDefinitions;
        nextLine();
    }

    private void nextLine()
    {
        do
        {
            lineRemaining = reader.ReadLine();
            ++LineNumber;
            Position = 0;
        } while (lineRemaining != null && lineRemaining.Length == 0);
    }

    public bool Next()
    {
        if (lineRemaining == null)
            return false;
        foreach (var def in tokenDefinitions)
        {
            var matched = def.Matcher.Match(lineRemaining);
            if (matched > 0)
            {
                Position += matched;
                Token = def.Token;
                TokenContents = lineRemaining.Substring(0, matched);
                lineRemaining = lineRemaining.Substring(matched);
                if (lineRemaining.Length == 0)
                    nextLine();

                return true;
            }
        }
        throw new Exception(string.Format("Unable to match against any tokens at line {0} position {1} \"{2}\"",
                                          LineNumber, Position, lineRemaining));
    }

    public string TokenContents { get; private set; }
    public object Token   { get; private set; }
    public int LineNumber { get; private set; }
    public int Position   { get; private set; }

    public void Dispose() => reader.Dispose();
}

示例程序:

string sample = @"( one (two 456 -43.2 "" \"" quoted"" ))";

var defs = new TokenDefinition[]
{
    // Thanks to [steven levithan][2] for this great quoted string
            // regex
    new TokenDefinition(@"([""'])(?:\\\1|.)*?\1", "QUOTED-STRING"),
    // Thanks to http://www.regular-expressions.info/floatingpoint.html
    new TokenDefinition(@"[-+]?\d*\.\d+([eE][-+]?\d+)?", "FLOAT"),
    new TokenDefinition(@"[-+]?\d+", "INT"),
    new TokenDefinition(@"#t", "TRUE"),
    new TokenDefinition(@"#f", "FALSE"),
    new TokenDefinition(@"[*<>\?\-+/A-Za-z->!]+", "SYMBOL"),
    new TokenDefinition(@"\.", "DOT"),
    new TokenDefinition(@"\(", "LEFT"),
    new TokenDefinition(@"\)", "RIGHT"),
    new TokenDefinition(@"\s", "SPACE")
};

TextReader r = new StringReader(sample);
Lexer l = new Lexer(r, defs);
while (l.Next())
    Console.WriteLine("Token: {0} Contents: {1}", l.Token, l.TokenContents);

输出:

Token: LEFT Contents: (
Token: SPACE Contents:
Token: SYMBOL Contents: one
Token: SPACE Contents:
Token: LEFT Contents: (
Token: SYMBOL Contents: two
Token: SPACE Contents:
Token: INT Contents: 456
Token: SPACE Contents:
Token: FLOAT Contents: -43.2
Token: SPACE Contents:
Token: QUOTED-STRING Contents: " \" quoted"
Token: SPACE Contents:
Token: RIGHT Contents: )
Token: RIGHT Contents: )

关于c# - 穷人的 C# "lexer"，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/673113/