fluent/fluentd · error · Fluent::ConfigError

format should be Regexp, need //, in #{key}: '#{format}'

Error message

format should be Regexp, need //, in #{key}: '#{format}'

What it means

MultilineParser#check_format_regexp (lib/fluent/plugin/parser_multiline.rb:147) requires every formatN and format_firstline value to be a Ruby regexp literal wrapped in slashes; anything else raises Fluent::ConfigError. The parser strips the first and last characters and compiles the middle, so an unslashed pattern (which may even be valid text) is treated as malformed input rather than a regex.

Source

Thrown at lib/fluent/plugin/parser_multiline.rb:147

      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. Wrap each formatN / format_firstline value in '/.../', e.g. format1 /^\\d{4}-/.
2. Trim stray whitespace after the closing slash.
3. Run fluentd --dry-run to verify before restart.

Example fix

# before
format1 ^\d{4}-\d{2}-\d{2}
# after
format1 /^\d{4}-\d{2}-\d{2}/

conf.each do |k, v|
  next unless k.start_with?('format') || k == 'format_firstline'
  abort "#{k} lacks // delimiters" unless v.start_with?('/') && v.end_with?('/')
end

def slashed_regexp_literal?(s)
  s.is_a?(String) && s.length >= 2 && s.start_with?('/') && s.end_with?('/')
end

When it happens

Trigger: format1 ^\d{4}- (pattern without surrounding '/'); format_firstline ^\[\d{4}\]; quoting a plain string like format1 '[INFO]'; trailing whitespace after the closing slash leaving the last char as a space.

Common situations: Users pasting raw patterns from docs into fluent.conf; transitioning from v0 formats where some directives accepted bare strings; configs where the trailing '/' was dropped during YAML-to-v1 conversion or templating.

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}: #{
• Invalid regexp in #{key}: #{e}
• unknown value conversion for key:'#{field_name}', type:'#{ty

AI-assisted analysis of fluent/fluentd@dd45c6e18d (2026-08-21). Data as JSON: /api/errors/0b07433843952617. Report an issue: GitHub.