Java: splitting a comma-separated string but ignoring commas in quotes

I have a string vaguely like this:

foo,bar,c;qual="baz,blurb",d;junk="quux,syzygy"

that I want to split by commas — but I need to ignore commas in quotes. How can I do this? Seems like a regexp approach fails; I suppose I can manually scan and enter a different mode when I see a quote, but it would be nice to use preexisting libraries. (edit: I guess I meant libraries that are already part of the JDK or already part of a commonly-used libraries like Apache Commons.)

the above string should split into:

foo
bar
c;qual="baz,blurb"
d;junk="quux,syzygy"

note: this is NOT a CSV file, it’s a single string contained in a file with a larger overall structure

Answer

Try:

public class Main { 
    public static void main(String[] args) {
        String line = "foo,bar,c;qual="baz,blurb",d;junk="quux,syzygy"";
        String[] tokens = line.split(",(?=(?:[^"]*"[^"]*")*[^"]*$)", -1);
        for(String t : tokens) {
            System.out.println("> "+t);
        }
    }
}

Output:

> foo
> bar
> c;qual="baz,blurb"
> d;junk="quux,syzygy"

In other words: split on the comma only if that comma has zero, or an even number of quotes ahead of it.

Or, a bit friendlier for the eyes:

public class Main { 
    public static void main(String[] args) {
        String line = "foo,bar,c;qual="baz,blurb",d;junk="quux,syzygy"";
        
        String otherThanQuote = " [^"] ";
        String quotedString = String.format(" " %s* " ", otherThanQuote);
        String regex = String.format("(?x) "+ // enable comments, ignore white spaces
                ",                         "+ // match a comma
                "(?=                       "+ // start positive look ahead
                "  (?:                     "+ //   start non-capturing group 1
                "    %s*                   "+ //     match 'otherThanQuote' zero or more times
                "    %s                    "+ //     match 'quotedString'
                "  )*                      "+ //   end group 1 and repeat it zero or more times
                "  %s*                     "+ //   match 'otherThanQuote'
                "  $                       "+ // match the end of the string
                ")                         ", // stop positive look ahead
                otherThanQuote, quotedString, otherThanQuote);

        String[] tokens = line.split(regex, -1);
        for(String t : tokens) {
            System.out.println("> "+t);
        }
    }
}

which produces the same as the first example.

EDIT

As mentioned by @MikeFHay in the comments:

I prefer using Guava’s Splitter, as it has saner defaults (see discussion above about empty matches being trimmed by String#split(), so I did:

Splitter.on(Pattern.compile(",(?=(?:[^"]*"[^"]*")*[^"]*$)"))