elastic/elasticsearch · error · RuntimeException

Invalid Mapping Rule : [{rule}]. Only a single character is

Error message

Invalid Mapping Rule : [{rule}]. Only a single character is allowed.

What it means

After a type_table rule matches the regex, the left-hand side is run through parseString (handling escapes) and must yield exactly one character. If lhs.length() != 1 parseTypes throws RuntimeException. Multi-character LHS is structurally invalid because each rule maps a single char to a single byte type.

Source

Thrown at modules/analysis-common/src/main/java/org/elasticsearch/analysis/common/WordDelimiterTokenFilterFactory.java:121

        return 0;
    }

    // source => type
    private static final Pattern typePattern = Pattern.compile("(.*)\\s*=>\\s*(.*)\\s*$");

    /**
     * parses a list of MappingCharFilter style rules into a custom byte[] type table
     */
    static byte[] parseTypes(Collection<String> rules) {
        SortedMap<Character, Byte> typeMap = new TreeMap<>();
        for (String rule : rules) {
            Matcher m = typePattern.matcher(rule);
            if (m.find() == false) {
                throw new RuntimeException("Invalid Mapping Rule : [" + rule + "]");
            }
            String lhs = parseString(m.group(1).trim());
            Byte rhs = parseType(m.group(2).trim());
            if (lhs.length() != 1) throw new RuntimeException("Invalid Mapping Rule : [" + rule + "]. Only a single character is allowed.");
            if (rhs == null) throw new RuntimeException("Invalid Mapping Rule : [" + rule + "]. Illegal type.");
            typeMap.put(lhs.charAt(0), rhs);
        }

        // ensure the table is always at least as big as DEFAULT_WORD_DELIM_TABLE for performance
        byte types[] = new byte[Math.max(typeMap.lastKey() + 1, WordDelimiterIterator.DEFAULT_WORD_DELIM_TABLE.length)];
        for (int i = 0; i < types.length; i++) {
            types[i] = WordDelimiterIterator.getType(i);
        }
        for (Map.Entry<Character, Byte> mapping : typeMap.entrySet()) {
            types[mapping.getKey()] = mapping.getValue();
        }
        return types;
    }

    private static Byte parseType(String s) {
        if (s.equals("LOWER")) return WordDelimiterFilter.LOWER;
        else if (s.equals("UPPER")) return WordDelimiterFilter.UPPER;

View on GitHub (pinned to db6a809a66)

Solutions

1. Provide exactly one character (or one escape sequence) on the LHS of each rule.
2. For multi-character patterns use a different mechanism (protected words, custom analyzer), not type_table.
3. Re-validate each entry's LHS length.

Example fix

// before
"type_table": ["abc => ALPHA"]
// after
"type_table": ["a => ALPHA", "b => ALPHA", "c => ALPHA"]

// After resolving escapes, each LHS must be exactly one char
static List<String> badLhs(List<String> typeTable) {
  return typeTable.stream()
    .map(r -> r.split("=>", 2)[0].trim())
    .filter(lhs -> resolveEscapes(lhs).length() != 1)
    .toList();
}

When it happens

Trigger: A type_table entry whose LHS decodes to two or more characters: "abc => LOWER", ".. => DIGIT", or an escape that resolves to a surrogate pair.

Common situations: Trying to map a whole word or punctuation sequence to a type; misunderstanding that type_table is per-character.

Related errors

• Invalid Mapping Rule : [{rule}]. Illegal type.
• Invalid Mapping Rule : [{rule}]
• Invalid escaped char in [{s}]
• length parameter must be provided
• Token filter [{name()}] cannot be used to parse synonyms

AI-assisted analysis of elastic/elasticsearch@db6a809a66 (2026-08-12). Data as JSON: /api/errors/1dd7a14fd6ac2cff. Report an issue: GitHub.