Regex - Escape escape characters

Question

My problem is quite complex, but can be boiled down to a simple example.

I am writing a custom query language where users can input strings which I parse to LinQ Expressions.

What I would like to able to do is to split strings by the * character, unless it is correctly escaped.

Input         Output                          Query Description
"*\\*"    --> { "*", "\\", "*" }       -- contains a '\'
"*\\\**"  --> { "*", "\\\*", "*" }     -- contains '\*'
"*\**"    --> { "*", "\*", "*" }       -- contains '*' (works now)

I don't mind Regex.Split returning empty strings, but I end up with this:

Regex.Split(@"*\\*", @"(?<!\\)(\*)")  --> {"", "*", "\\*"}

As you can see, I have tried with negative lookbehind, which works for all my cases except this one. I have also tried Regex.Escape, but with no luck.

Obviously, my problem is that I am looking for \*, which \\* matches. But in this case, \\ is another escaped sequence.

Any solution doesn't necessary have to involve a Regex.

Answer 1

I think it's much easier to match than to split, especially since you are not removing anything from the initial string. So what to match? Everything except an unescaped *.

How to do that? With the below regex:

@"(?:[^*\\]+|\\.)+|\*"

(?:[^*\\]+|\\.)+ matches everything that is not a *, or any escaped character. No need for any lookaround.

\* will match the separator.

In code:

using System;
using System.Text.RegularExpressions;
using System.Linq;
public class Test
{
    public static void Main()
    {   
        string[] tests = new string[]{
            @"*\\*",
            @"*\\\**",
            @"*\**",
        };

        Regex re = new Regex(@"(?:[^*\\]+|\\.)+|\*");

        foreach (string s in tests) {
            var parts = re.Matches(s)
             .OfType<Match>()
             .Select(m => m.Value)
             .ToList();

            Console.WriteLine(string.Join(", ", parts.ToArray()));
        }
    }
}

Output:

*, \\, *
*, \\\*, *
*, \*, *

ideone demo

Answer 2

I've came up with this regexp (?<=(?:^|[^\\])(?:\\\\)*)(\*).

Explanation:

You just white-list situations that can happen before * and these are:

• start of the string ^
• not \ - [^\\]
• (not \ or beginning of the string) and then even number of \ - (^|[^\\])(\\\\)*

Test code and examples:

string[] tests = new string[]{
    @"*\\*",
    @"*\\\**",
    @"*\**",
    @"test\**test2",
};

Regex re = new Regex(@"(?<=(?:^|[^\\])(?:\\\\)*)(\*)");

foreach (string s in tests) {
    string[] m = re.Split( s );
    Console.WriteLine(String.Format("{0,-20} {1}", s, String.Join(", ",
       m.Where(x => !String.IsNullOrEmpty(x)))));
}

Result:

*\\*                 *, \\, *
*\\\**               *, \\\*, *
*\**                 *, \*, *
test\**test2         test\*, *, test2