CSV Parsing Rules

CSV: parsing rules that bite

Real CSV (RFC 4180) has three rules beginners miss:

1. Quoted fields — "hello, world" is ONE field containing a comma.
2. Escaped quotes — "" inside quotes is a literal ".
3. Newlines in quotes — a row can span lines.

A split(",") parser fails all three:

// naive — breaks on quoted commas
String[] fields = line.split(",");

// character scanner — the honest parser
List<String> parseRow(String line) {
    List<String> out = new ArrayList<>();
    StringBuilder cur = new StringBuilder();
    boolean inQuotes = false;
    for (int i = 0; i < line.length(); i++) {
        char c = line.charAt(i);
        if (inQuotes) {
            if (c == '"') {
                if (i + 1 < line.length() && line.charAt(i + 1) == '"') { cur.append('"'); i++; }
                else inQuotes = false;
            } else cur.append(c);
        } else if (c == '"') inQuotes = true;
        else if (c == ',') { out.add(cur.toString()); cur.setLength(0); }
        else cur.append(c);
    }
    out.add(cur.toString());
    return out;
}

The scanner handles rules 1–2; full rule 3 needs row-merging across lines — beyond today's scope, but you now know why libraries exist.