How to split a comma separated String while ignoring escaped commas?

Question

I need to write a extended version of the StringUtils.commaDelimitedListToStringArray function which gets an additional parameter: the escape char.

so calling my:

Answer 1

As matt b said, [^\\], will interpret the character preceding the comma as a part of the delimiter.

"test\\\\\\,test\\\\,test\\,test,test"
  -(split)->
["test\\\\\\,test\\\\,test\\,tes" , "test"]

As drvdijk said, (? will misinterpret escaped backslashes.

"test\\\\\\,test\\\\,test\\,test,test"
  -(split)->
["test\\\\\\,test\\\\,test\\,test" , "test"]
  -(unescape commas)->
["test\\\\,test\\,test,test" , "test"]

I would expect being able to escape backslashes as well...

"test\\\\\\,test\\\\,test\\,test,test"
  -(split)->
["test\\\\\\,test\\\\" , "test\\,test" , "test"]
  -(unescape commas and backslashes)->
["test\\,test\\" , "test,test" , "test"]

drvdijk suggested (?<=(? which works well for lists with elements ending with up to 100 backslashes. This is far enough... but why a limit? Is there a more efficient way (isn't lookbehind greedy)? What about invalid strings?

I searched for a while for a generic solution, then I wrote the thing myself... The idea is to split following a pattern that matches the list elements (instead of matching the delimiter).

My answer does not take the escape character as a parameter.

public static List commaDelimitedListStringToStringList(String list) {
    // Check the validity of the list
    // ex: "te\\st" is not valid, backslash should be escaped
    if (!list.matches("^(([^\\\\,]|\\\\,|\\\\\\\\)*(,|$))+")) {
        // Could also raise an exception
        return null;
    }
    // Matcher for the list elements
    Matcher matcher = Pattern
            .compile("(?<=(^|,))([^\\\\,]|\\\\,|\\\\\\\\)*(?=(,|$))")
            .matcher(list);
    ArrayList result = new ArrayList();
    while (matcher.find()) {
        // Unescape the list element
        result.add(matcher.group().replaceAll("\\\\([\\\\,])", "$1"));
    }
    return result;
}

Description for the pattern (unescaped):

(?<=(^|,)) forward is start of string or a ,

([^\\,]|\\,|\\\\)* the element composed of \,, \\ or characters wich are neither \ nor ,

(?=(,|$)) behind is end of string or a ,

The pattern may be simplified.

Even with the 3 parsings (matches + find + replaceAll), this method seems faster than the one suggested by drvdijk. It can still be optimized by writing a specific parser.

Also, what is the need of having an escape character if only one character is special, it could simply be doubled...

public static List commaDelimitedListStringToStringList2(String list) {
    if (!list.matches("^(([^,]|,,)*(,|$))+")) {
        return null;
    }
    Matcher matcher = Pattern.compile("(?<=(^|,))([^,]|,,)*(?=(,|$))")
                    .matcher(list);
    ArrayList result = new ArrayList();
    while (matcher.find()) {
        result.add(matcher.group().replaceAll(",,", ","));
    }
    return result;
}