apache/druid · error · RE

Failed to parse expression: %s

Error message

Failed to parse expression: %s

What it means

Parser.parse walks the ANTLR parse tree and builds the Expr AST; if the listener produces no AST (parsed == null), the input could not be parsed into a valid expression and RE (RuntimeException) 'Failed to parse expression: <in>' is thrown. This indicates syntactically invalid or semantically empty expression text.

Source

Thrown at processing/src/main/java/org/apache/druid/math/expr/Parser.java:142

  {
    return parse(in, macroTable, true);
  }


  @VisibleForTesting
  public static Expr parse(String in, ExprMacroTable macroTable, boolean withFlatten)
  {
    ExprLexer lexer = new ExprLexer(new ANTLRInputStream(in));
    CommonTokenStream tokens = new CommonTokenStream(lexer);
    ExprParser parser = new ExprParser(tokens);
    parser.setBuildParseTree(true);
    ParseTree parseTree = parser.expr();
    ParseTreeWalker walker = new ParseTreeWalker();
    ExprListenerImpl listener = new ExprListenerImpl(parseTree, macroTable);
    walker.walk(listener, parseTree);
    Expr parsed = listener.getAST();
    if (parsed == null) {
      throw new RE("Failed to parse expression: %s", in);
    }
    return withFlatten ? flatten(parsed) : parsed;
  }

  /**
   * Create an {@link IdentifierExpr} for some identifier
   */
  public static Expr identifier(String identifier)
  {
    return new IdentifierExpr(identifier);
  }

  /**
   * Create a {@link StringExpr} for a string constant
   */
  public static Expr constant(String constant)
  {
    return new StringExpr(constant);

View on GitHub (pinned to 9b90983fd2)

Solutions

1. Inspect the expression string for syntax errors (unbalanced parens, stray characters) and fix it.
2. Validate the expression with a quick Parser.parse test or the web console expression editor before deploying the spec.
3. Check any macros in the expression expand to valid text.
4. Wrap parsing in try-catch if user-supplied expressions are accepted, returning a clear validation message.

Example fix

// before
String in = "concat(x, )"; // invalid
Expr e = Parser.parse(in, null);
// after
String in = "concat(x, 'y')";
Expr e = Parser.parse(in, null);

try { Parser.parse(exprString, null); } catch (Exception e) { /* reject invalid expression up front */ }

try { Expr e = Parser.parse(in, macroTable); } catch (RuntimeException e) { if (e.getMessage().startsWith("Failed to parse expression")) { /* surface user-friendly syntax error */ } }

When it happens

Trigger: Calling Parser.parse(in, macroTable) or Parser.parse(in, flatten) with malformed input — unbalanced parentheses, unknown tokens, empty string after macro substitution, or invalid operator sequences that ANTLR recovers from but yield no AST.

Common situations: Typos in virtual-column/dimension expressions in ingestion specs or query filters; SQL-string quoting issues leaving stray characters; macros expanding to empty/invalid text; hand-edited JSON specs.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• failed to parse address
• failed to parse address
• Expression %s has non-constant inputs.
• Function[%s] %s
• Unable to transform apply function:[%s]

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/1235e19a8af2ad52. Report an issue: GitHub.