Important: This documentation is about an older version. It's relevant only to the release noted, many of the features and functions have been updated or replaced. Please view the current version.

Open source


The regex stage is a parsing stage that parses a log line using a regular expression. Named capture groups in the regex support adding data into the extracted map.


  # The RE2 regular expression. Each capture group must be named.
  expression: <string>

  # Name from extracted data to parse. If empty, uses the log message.
  [source: <string>]

expression needs to be a Go RE2 regex string. Every capture group (re) will be set into the extracted map, every capture group must be named: (?P<name>re). The name of the capture group will be used as the key in the extracted map.

Because of how YAML treats backslashes in double-quoted strings, note that all backslashes in a regex expression must be escaped when using double quotes. For example, all of these are valid:

  • expression: \w*
  • expression: '\w*'
  • expression: "\\w*"

But these are not:

  • expression: \\w* (only escape backslashes when using double quotes)
  • expression: '\\w*' (only escape backslashes when using double quotes)
  • expression: "\w*" (backslash must be escaped)

If you run Promtail with the --config.expand-env=true flag the configuration will run through envsubst which will replace double slashes with single slashes. Because of this when using expand-env=true you need to use double slashes for each single slash. For example:

  • expression: '\w*' must be expression: '\\w*'
  • expression: "\\w*" must be expression: "\\\\w*"


Without source

Given the pipeline:

- regex:
    expression: "^(?s)(?P<time>\\S+?) (?P<stream>stdout|stderr) (?P<flags>\\S+?) (?P<content>.*)$"

And the log line:

2019-01-01T01:00:00.000000001Z stderr P i'm a log message!

The following key-value pairs would be added to the extracted map:

  • time: 2019-01-01T01:00:00.000000001Z,
  • stream: stderr,
  • flags: P,
  • content: i'm a log message

With source

Given the pipeline:

- json:
- regex:
    expression: "^(?P<year>\\d+)"
    source:     "time"

And the log line:


The first stage would add the following key-value pairs into the extracted map:

  • time: 2019-01-01T01:00:00.000000001Z

While the regex stage would then parse the value for time in the extracted map and append the following key-value pairs back into the extracted map:

  • year: 2019