C# Extension Method - String Split that also accepts an Escape Character

Question

I'd like to write an extension method for the .NET String class. I'd like it to be a special varation on the Split method - one that takes an escape character to prevent splitting the string when a escape character is used before the separator.

What's the best way to write this? I'm curious about the best non-regex way to approach it.
Something with a signature like...

public static string[] Split(this string input, string separator, char escapeCharacter)
{
   // ...
}

UPDATE: Because it came up in one the comments, the escaping...

In C# when escaping non-special characters you get the error - CS1009: Unrecognized escape sequence.

In IE JScript the escape characters are throw out. Unless you try \u and then you get a "Expected hexadecimal digit" error. I tested Firefox and it has the same behavior.

I'd like this method to be pretty forgiving and follow the JavaScript model. If you escape on a non-separator it should just "kindly" remove the escape character.

Solution 1

How about:

public static IEnumerable<string> Split(this string input, 
                                        string separator,
                                        char escapeCharacter)
{
    int startOfSegment = 0;
    int index = 0;
    while (index < input.Length)
    {
        index = input.IndexOf(separator, index);
        if (index > 0 && input[index-1] == escapeCharacter)
        {
            index += separator.Length;
            continue;
        }
        if (index == -1)
        {
            break;
        }
        yield return input.Substring(startOfSegment, index-startOfSegment);
        index += separator.Length;
        startOfSegment = index;
    }
    yield return input.Substring(startOfSegment);
}

That seems to work (with a few quick test strings), but it doesn't remove the escape character - that will depend on your exact situation, I suspect.

Solution 2

This will need to be cleaned up a bit, but this is essentially it....

List<string> output = new List<string>();
for(int i=0; i<input.length; ++i)
{
    if (input[i] == separator && (i==0 || input[i-1] != escapeChar))
    {
        output.Add(input.substring(j, i-j);
        j=i;
    }
}

return output.ToArray();

Solution 3

Here is solution if you want to remove the escape character.

public static IEnumerable<string> Split(this string input, 
                                        string separator, 
                                        char escapeCharacter) {
    string[] splitted = input.Split(new[] { separator });
    StringBuilder sb = null;

    foreach (string subString in splitted) {
        if (subString.EndsWith(escapeCharacter.ToString())) {
            if (sb == null)
                sb = new StringBuilder();
            sb.Append(subString, 0, subString.Length - 1);
        } else {
            if (sb == null)
                yield return subString;
            else {
                sb.Append(subString);
                yield return sb.ToString();
                sb = null;
            }
        }
    }
    if (sb != null)
        yield return sb.ToString();
}

Solution 4

My first observation is that the separator ought to be a char not a string since escaping a string using a single character may be hard -- how much of the following string does the escape character cover? Other than that, @James Curran's answer is pretty much how I would handle it - though, as he says it needs some clean up. Initializing j to 0 in the loop initializer, for instance. Figuring out how to handle null inputs, etc.

You probably want to also support StringSplitOptions and specify whether empty string should be returned in the collection.

Solution 5

You can try something like this. Although, I would suggest implementing with unsafe code for performance critical tasks.

public static class StringExtensions
{
    public static string[] Split(this string text, char escapeChar, params char[] seperator)
    {
        return Split(text, escapeChar, seperator, int.MaxValue, StringSplitOptions.None);
    }

    public static string[] Split(this string text, char escapeChar, char[] seperator, int count)
    {
        return Split(text, escapeChar, seperator, count, StringSplitOptions.None);
    }

    public static string[] Split(this string text, char escapeChar, char[] seperator, StringSplitOptions options)
    {
        return Split(text, escapeChar, seperator, int.MaxValue, options);
    }

    public static string[] Split(this string text, char escapeChar, char[] seperator, int count, StringSplitOptions options)
    {
        if (text == null)
        {
            throw new ArgumentNullException("text");
        }

        if (text.Length == 0)
        {
            return new string[0];
        }

        var segments = new List<string>();

        bool previousCharIsEscape = false;
        var segment = new StringBuilder();

        for (int i = 0; i < text.Length; i++)
        {
            if (previousCharIsEscape)
            {
                previousCharIsEscape = false;

                if (seperator.Contains(text[i]))
                {
                    // Drop the escape character when it escapes a seperator character.
                    segment.Append(text[i]);
                    continue;
                }

                // Retain the escape character when it escapes any other character.
                segment.Append(escapeChar);
                segment.Append(text[i]);
                continue;
            }

            if (text[i] == escapeChar)
            {
                previousCharIsEscape = true;
                continue;
            }

            if (seperator.Contains(text[i]))
            {
                if (options != StringSplitOptions.RemoveEmptyEntries || segment.Length != 0)
                {
                    // Only add empty segments when options allow.
                    segments.Add(segment.ToString());
                }

                segment = new StringBuilder();
                continue;
            }

            segment.Append(text[i]);
        }

        if (options != StringSplitOptions.RemoveEmptyEntries || segment.Length != 0)
        {
            // Only add empty segments when options allow.
            segments.Add(segment.ToString());
        }

        return segments.ToArray();
    }
}

View more solutions

Comments

[tvanfosson]

It looks like you're assuming that anytime the escape character appears it's followed by the separator string. What if it isn't?

[Jon Skeet]

I'm only going on what's in the question - if the escape character appears before the separator, it should prevent that separator from being used for splitting. I don't try to remove the escape character or process it in any other way. Naive, perhaps, but that's all the information we've got.

[BuddyJoe]

nice catch. I'll go fix that in the original question.

[missaghi]

cool, what is the benefit of ienumberable over returning a string array?

[Jon Skeet]

Deferred execution and streaming - we don't need to buffer everything up.

[BuddyJoe]

Jon, updated the question (top) to include the escape removal question. Never thought of the "yield" strategy... interesting. +1

[rjrapson]

After the split call, wouldn't you replace g with just the separator and not include the escape? That would save you the trouble of having to remove the escape from the returned string.

[BuddyJoe]

This is the classic "placeholder" pattern. I like the use of the GUID as the placeholder. I would say that this is good enough for "hobby" code, but not "Global Thermonuclear War" code.

[Jon Skeet]

@tvanfosson: In my experience escape character semantics vary considerably. Should it translate \n into a linefeed, for example? That's way beyond the scope of a splitting method, IMO.

[Jon Skeet]

@Bruno: I would handle unescaping in a separate method, particularly if the escape character is going to be used for more than just "don't escape the separator". It can get quite involved. Having said that, if the escape character escapes itself, it could get tricky. e.g. "foo\\,bar" is "foo\" "bar"

[Jon Skeet]

(Assuming a '\' escape character and a "," separator.)

[BFree]

@rjrapson: Good point. I guess it depends on what the OP wanted. I guess you can extend this method to take a bool whether or not to include the escape character. @Bruno: The only real issue I see with this approach, is that a Guid includes a "-" which CAN be the separator.

[BuddyJoe]

I'm a little green on parsing, but shouldn't the escape character put the "state" into a special mode for one character only. Then once you pass this one character, return back to regular mode. Then \\, situations are not that tricky. \\ would turn into \ and the separator , would be processed.

[BuddyJoe]

Thanks for all the input. I might consider the unescaping in a separate method. Especially, if it makes the code more readable/maintainable.

[Jon Skeet]

@Bruno: Your "state" comment is right, if an escape character can escape itself. Basically it will all depend on what your escaping requirements.

[innominate227]

two of your overloads take count but its not used