quote value ending with multi-character delimiter prefix

src/main/java/org/apache/commons/csv/CSVFormat.java

@@ -2097,6 +2097,33 @@ private boolean isDelimiter(final char ch0, final CharSequence charSeq, final in
         return true;
     }
 
+    /**
+     * Tests whether appending the delimiter after {@code charSeq} would let the parser match the delimiter starting inside the value. This happens with a
+     * multi-character delimiter when the value ends with a straddling prefix of it (for delimiter {@code ||}, a value ending in {@code |} followed by the
+     * delimiter yields {@code |||}, which the greedy lexer splits one character early). Such a value must be encapsulated so the field boundary is unambiguous.
+     */
+    private boolean endsWithDelimiterPrefix(final CharSequence charSeq, final char[] delimiter, final int delimiterLength) {
+        if (delimiterLength < 2) {
+            return false;
+        }
+        final int len = charSeq.length();
+        for (int start = Math.max(0, len - delimiterLength + 1); start < len; start++) {
+            boolean match = true;
+            for (int j = 0; j < delimiterLength; j++) {
+                final int idx = start + j;
+                final char c = idx < len ? charSeq.charAt(idx) : delimiter[idx - len];
+                if (c != delimiter[j]) {
+                    match = false;
+                    break;
+                }
+            }
+            if (match) {
+                return true;
+            }
+        }
+        return false;
+    }
+
     /**
      * Tests whether escapes are being processed.
      *
@@ -2510,6 +2537,9 @@ private void printWithQuotes(final Object object, final CharSequence charSeq, fi
                         // encapsulate if we end in anything less than ' '
                         if (isTrimChar(c)) {
                             quote = true;
+                        } else if (endsWithDelimiterPrefix(charSeq, delim, delimLength)) {
+                            // A trailing partial multi-character delimiter would merge with the following delimiter on read.
+                            quote = true;
                         }
                     }
                 }

src/test/java/org/apache/commons/csv/CSVPrinterTest.java

@@ -1908,6 +1908,28 @@ void testQuoteCommentMarkerFirstChar() throws IOException {
         }
     }
 
+    @Test
+    void testQuoteValueEndingWithMultiCharacterDelimiterPrefix() throws IOException {
+        final CSVFormat format = CSVFormat.DEFAULT.builder().setDelimiter("||").get();
+        final StringWriter sw = new StringWriter();
+        try (CSVPrinter printer = new CSVPrinter(sw, format)) {
+            // "a|" ends with the delimiter's first char; unquoted output "a|||b" would split one char early on read.
+            printer.printRecord("a|", "b");
+            // "a|b" does not end with a delimiter prefix, so it stays unquoted.
+            printer.printRecord("a|b", "c");
+        }
+        final String string = sw.toString();
+        assertEquals("\"a|\"||b" + RECORD_SEPARATOR + "a|b||c" + RECORD_SEPARATOR, string);
+        try (CSVParser parser = CSVParser.parse(string, format)) {
+            final List<CSVRecord> records = parser.getRecords();
+            assertEquals(2, records.size());
+            assertEquals("a|", records.get(0).get(0));
+            assertEquals("b", records.get(0).get(1));
+            assertEquals("a|b", records.get(1).get(0));
+            assertEquals("c", records.get(1).get(1));
+        }
+    }
+
     @Test
     void testQuoteNonNumeric() throws IOException {
         final StringWriter sw = new StringWriter();