regex

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.

Schema

regex:
  # 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 backslashes with single backslashes. Because of this when using expand-env=true you need to use double backslashes for each single backslash. For example:

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

Example

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

And the log line:

{"time":"2019-01-01T01:00:00.000000001Z"}

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: