Patterns for custom formats

A pattern is a string in which text is matched literally and {name:type} extracts a field. It compiles to a finite state machine, not a regular expression, so catastrophic backtracking on bad input cannot happen.

const parser = createParser({
  pattern: '{ts:iso} {level:level} {service:word}: {msg:rest}'
});
parser.parseLine('2026-10-01T09:14:07Z ERROR db: connection reset');
// { ts: Date, level: 'error', fields: { service: 'db' }, msg: 'connection reset' }

Field types#

Type	Reads	Result
`iso`	ISO 8601 date and time	`Date`
`level`	`trace`, `debug`, `info`, `warn`, `error`, `fatal` and common synonyms	lower-case string
`int`, `float`	numbers	`number`
`word`	up to the next space	string
`quoted`	text in double quotes	string without quotes
`ip`	IPv4 or IPv6	string
`rest`	everything to the end of the line	string

Fields named ts, level and msg land in the properties of the same name; the others go to rec.fields.

Optional parts#

A ? after the type makes the field optional together with the space before it:

text

{ts:iso} {level:level} {trace:word?} {msg:rest}

2026-10-01T09:14:07Z INFO started parses, and trace stays undefined.

Rules#

rest is only allowed at the end of a pattern.
Two adjacent fields without a separator are rejected, because it is unclear where the first one ends. Sverlo reports it when the parser is created.
A literal { is written {{.

Check a pattern

Run a dozen real lines through sverlo parse --pattern '...' --limit 10 and look at parser.errors. A pattern that swallows everything into rest usually hides a mistake in the format.

Next: filtering the records, see the query language.