fluent/fluentd · error · Fluent::ConfigError

Invalid regexp in #{key}: #{e}

Error message

Invalid regexp in #{key}: #{e}

What it means

MultilineParser#check_format_regexp (lib/fluent/plugin/parser_multiline.rb:144) compiles an individual formatN or format_firstline value (delimiters '/' stripped) with Regexp::MULTILINE and wraps any RegexpError in Fluent::ConfigError. Unlike error 230 (which validates the joined regex), this validates each piece separately, pinpointing the exact key whose regex body is invalid Ruby regexp syntax.

Source

Thrown at lib/fluent/plugin/parser_multiline.rb:144

        }
      end

      def check_format_range(conf)
        invalid_formats = conf.keys.select { |k|
          m = k.match(/^format(\d+)$/)
          m ? !((1..FORMAT_MAX_NUM).include?(m[1].to_i)) : false
        }
        unless invalid_formats.empty?
          raise Fluent::ConfigError, "Invalid formatN found. N should be 1 - #{FORMAT_MAX_NUM}: " + invalid_formats.join(",")
        end
      end

      def check_format_regexp(format, key)
        if format[0] == '/' && format[-1] == '/'
          begin
            Regexp.new(format[1..-2], Regexp::MULTILINE)
          rescue => e
            raise Fluent::ConfigError, "Invalid regexp in #{key}: #{e}"
          end
        else
          raise Fluent::ConfigError, "format should be Regexp, need //, in #{key}: '#{format}'"
        end
      end
    end
  end
end

View on GitHub (pinned to dd45c6e18d)

Solutions

1. Reproduce the compile error standalone: ruby -e "Regexp.new(File.read? 'body', Regexp::MULTILINE)" or paste the body into a Ruby shell.
2. Escape embedded '/' inside the body as '\\/' or restructure the pattern.
3. Validate configs in CI with fluentd --dry-run.
4. Prefer simple anchor-heavy patterns and test them against sample log lines with ruby's Regexp before deploying.

Example fix

# before
format_firstline /^2021\/\d{2/  # unbalanced brace -> RegexpError
# after
format_firstline /^2021\/\d{2}\//

# compile each formatN body before deploy
formats = ['/^\\d{4}-/', '(?<t>\\d{2}:\\d{2})']
formats.each { |body| Regexp.new(body, Regexp::MULTILINE) }

When it happens

Trigger: format3 /(?<a>\d+/ (unbalanced group); format_firstline /\d{4\/\d{2}/ with an unescaped '/' inside; quantifier mistakes like /a{,3}/ or /a**/; lookbehind constructs unsupported by fluentd's Ruby version.

Common situations: Hand-editing multiline formats for application logs; escaping regressions when configs pass through templating (Helm, ERB) that eats backslashes; regexes copied from PCRE-style tools (grep, Logstash) using syntax Ruby rejects.

Related errors

• Invalid regexp '#{formats}': #{e}
• Jump of format index found. format#{i - 1} is missing.
• Invalid formatN found. N should be 1 - #{FORMAT_MAX_NUM}: #{
• format should be Regexp, need //, in #{key}: '#{format}'
• invalid match - regex

AI-assisted analysis of fluent/fluentd@dd45c6e18d (2026-08-21). Data as JSON: /api/errors/0fdc4e0b5d512056. Report an issue: GitHub.