How to parse an escape sequence?

Question

I'm writing a parser for my own markup and I need to handle a few escape sequences but I'm not sure which strategy I should choose.

In particular I have two in my mind.

Here's an example foo \\\<bar baz with two of them: \\ and \<.

When I now scan the string char by char

1. should I detect the backslash \ and then check if the next character is an excapable one or
2. should I check for the character and then look back to see whether it's preceded by a backslash \?

Are there any major (dis)advantages in either one?

Answer 1

You need to know where you're at. The way to do that is a state machine. If you're only doing \r, \t, \n, \", and \\, you can get by with a very simple one. Like this (fiddle here):

public static class StringExtensions
{
    private enum UnescapeState
    {
        Unescaped,
        Escaped
    }

    public static String Unescape(this String s)
    {
        var sb = new System.Text.StringBuilder();
        UnescapeState state = UnescapeState.Unescaped;

        foreach (var ch in s)
        {
            switch (state)
            {
                case UnescapeState.Escaped:
                    switch (ch)
                    {
                        case 't':
                            sb.Append('\t');
                            break;
                        case 'n':
                            sb.Append('\n');
                            break;
                        case 'r':
                            sb.Append('\r');
                            break;
                        
                        case '\\':
                        case '\"':
                            sb.Append(ch);
                            break;

                        default:
                            throw new Exception("Unrecognized escape sequence '\\" + ch + "'");

                    }
                    state = UnescapeState.Unescaped;
                    break;

                case UnescapeState.Unescaped:
                    if (ch == '\\')
                    {
                        state = UnescapeState.Escaped;
                    }
                    else
                    {
                        sb.Append(ch);
                    }
                    break;
            }
        }

        if (state == UnescapeState.Escaped)
        {
            throw new Exception("Unterminated escape sequence");
        }

        return sb.ToString();
    }
}