Regular Expression To Split On Comma Except If Quoted

Question

What is the regular expression to split on comma (,) except if surrounded by double quotes? For example:

max,emily,john = ["max", "emily", "john"]

BUT

max,"emily,kate",john = ["max", "emily,kate", "john"]

Looking to use in C#: Regex.Split(string, "PATTERN-HERE");

Thanks.

Answer 1

Situations like this often call for something other than regular expressions. They are nifty, but patterns for handling this kind of thing are more complicated than they are useful.

You might try something like this instead:

public static IEnumerable<string> SplitCSV(string csvString)
{
    var sb = new StringBuilder();
    bool quoted = false;

    foreach (char c in csvString) {
        if (quoted) {
            if (c == '"')
                quoted = false;
            else
                sb.Append(c);
        } else {
            if (c == '"') {
                quoted = true;
            } else if (c == ',') {
                yield return sb.ToString();
                sb.Length = 0;
            } else {
                sb.Append(c);
            }
        }
    }

    if (quoted)
        throw new ArgumentException("csvString", "Unterminated quotation mark.");

    yield return sb.ToString();
}

It probably needs a few tweaks to follow the CSV spec exactly, but the basic logic is sound.

Answer 2

This is a clear-cut case for a CSV parser, so you should be using .NET's own CSV parsing capabilities or cdhowie's solution.

Purely for your information and not intended as a workable solution, here's what contortions you'd have to go through using regular expressions with Regex.Split():

You could use the regex (please don't!)

(?<=^(?:[^"]*"[^"]*")*[^"]*)  # assert that there is an even number of quotes before...
\s*,\s*                       # the comma to be split on...
(?=(?:[^"]*"[^"]*")*[^"]*$)   # as well as after the comma.

if your quoted strings never contain escaped quotes, and you don't mind the quotes themselves becoming part of the match.

This is horribly inefficient, a pain to read and debug, works only in .NET, and it fails on escaped quotes (at least if you're not using "" to escape a single quote). Of course the regex could be modified to handle that as well, but then it's going to be perfectly ghastly.