What causes "Could not parse `fileRegex` [%s]" in apache/druid?

Fix the fileRegex to be a valid Java regular expression; test it with Pattern.compile or a Java-regex validator.

apache/druid · error · IllegalArgumentException

Could not parse `fileRegex` [%s]

Error message

Could not parse `fileRegex` [%s]

What it means

After the mutually-exclusive checks, UriExtractionNamespace's constructor validates fileRegex by compiling it with Pattern.compile. A PatternSyntaxException is rethrown as this IAE (with the bad pattern and the cause attached), so malformed regexes fail fast at config load instead of at poll time.

Source

Thrown at extensions-core/lookups-cached-global/src/main/java/org/apache/druid/query/lookup/namespace/UriExtractionNamespace.java:133

      this.pollPeriod = Period.ZERO;
    } else {
      this.pollPeriod = pollPeriod;
    }
    this.fileRegex = fileRegex == null ? versionRegex : fileRegex;
    if (fileRegex != null && versionRegex != null) {
      throw new IAE("Cannot specify both versionRegex and fileRegex. versionRegex is deprecated");
    }

    if (uri != null && this.fileRegex != null) {
      throw new IAE("Cannot define both uri and fileRegex");
    }

    if (this.fileRegex != null) {
      try {
        Pattern.compile(this.fileRegex);
      }
      catch (PatternSyntaxException ex) {
        throw new IAE(ex, "Could not parse `fileRegex` [%s]", this.fileRegex);
      }
    }
    this.maxHeapPercentage = maxHeapPercentage == null ? DEFAULT_MAX_HEAP_PERCENTAGE : maxHeapPercentage;
  }

  public String getFileRegex()
  {
    return fileRegex;
  }

  public FlatDataParser getNamespaceParseSpec()
  {
    return this.namespaceParseSpec;
  }

  public URI getUri()
  {
    return uri;

View on GitHub (pinned to 9b90983fd2)

Solutions

Fix the fileRegex to be a valid Java regular expression; test it with Pattern.compile or a Java-regex validator.
Double JSON backslashes in escape sequences (e.g. ".*\\\\.csv" in JSON for ".*\\.csv" in Java).
Check the wrapped PatternSyntaxException message for the exact character index of the syntax error.
Simplify the pattern or use a broader prefix match if complex filtering is unnecessary.

Example fix

// before (invalid Java regex)
"fileRegex": "[a-z"
// after
"fileRegex": "[a-z]+\\.csv"

Defensive patterns

Strategy: validation

Validate before calling

// java
String regex = cfg.get("fileRegex").asText();
try { java.util.regex.Pattern.compile(regex); }
catch (java.util.regex.PatternSyntaxException e) {
  throw new IllegalArgumentException("bad fileRegex: " + e.getMessage());
}

Try / catch

// java
try { mapper.readValue(json, UriExtractionNamespace.class); }
catch (IllegalArgumentException e) {
  if (e.getMessage().contains("Could not parse `fileRegex`")) { /* fix pattern */ }
}

Prevention

Compile the pattern with Pattern.compile in a unit test before shipping configs.
Remember JSON string escaping: double backslashes for regex escapes.
Use Java-flavored regex syntax, not grep/PCRE extensions.

When it happens

Trigger: Providing a syntactically invalid fileRegex (e.g. "[a-", "*foo", unbalanced parentheses, bad escape like "\\\\" errors) in the UriExtractionNamespace config.

Common situations: Regexes written for grep/sed syntax rather than Java regex; unescaped dots/brackets in filename patterns; string-escaping mistakes in JSON where backslashes were consumed by the JSON parser, leaving an invalid Java pattern.

Related errors

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/09baaa8c8ff42f80. Report an issue: GitHub.