The EARS patterns

The classifier reads each requirement and assigns it one pattern. A requirement is built from a system, a shall response, and zero or more optional clauses: a trigger (WHEN), a state (WHILE), a condition (IF ... THEN), and a feature (WHERE). The pattern follows directly from which of those clauses are present.

There are five canonical patterns plus one combinator:

Pattern	Clause present
Ubiquitous	none
Event-driven	WHEN only
State-driven	WHILE only
Unwanted	IF ... THEN only
Optional	WHERE only
Complex	any other combination

When exactly one optional clause is present, the requirement takes the matching single-clause pattern. With no optional clauses it is ubiquitous. With more than one optional clause, or any combination the single-clause rules do not name, it is Complex.

The examples below are the finance options demo, references/finance_options/, which exercises every pattern in order.

Ubiquitous

An invariant the system always holds, with no qualifying clause.

FIN-001  The option pricer shall produce non-negative call prices.

The system is option pricer, the response is produce non-negative call prices. With no WHEN, WHILE, IF, or WHERE, the requirement is ubiquitous.

Event-driven

A response triggered by an event, introduced by WHEN.

FIN-002  WHEN market data arrives, the implied-volatility surface shall be within tolerance.

The trigger is market data arrives. Because the trigger is the only optional clause, the pattern is event-driven.

State-driven

A response that holds while the system is in a given state, introduced by WHILE.

FIN-003  WHILE the exchange is open, the portfolio delta shall be at most the limit.

The state is the exchange is open. With the state as the only optional clause, the pattern is state-driven.

Unwanted

A response to an unwanted condition, written as IF ... THEN .... The IF clause and its , THEN marker are required together.

FIN-004  IF an order breaches the position limit, THEN the order gateway shall reject the order.

The condition is an order breaches the position limit. With the condition as the only optional clause, the pattern is unwanted. Responses that begin with reject, raise, fail, or error are additionally marked as error rejections.

Optional

A response that applies only where a feature is enabled, introduced by WHERE.

FIN-005  WHERE same-day settlement is enabled, the settlement clock shall be bounded by the window.

The feature is same-day settlement is enabled. With the feature as the only optional clause, the pattern is optional.

Complex

Any requirement that combines more than one optional clause. The clauses are read left to right and may mix WHILE, WHEN, IF ... THEN, and WHERE.

FIN-006  WHILE the exchange is open, WHEN a quote changes, IF the quote is firm, THEN the call price curve shall be monotone-increasing.

This carries a state (the exchange is open), a trigger (a quote changes), and a condition (the quote is firm) on one system. Because three optional clauses are present, the pattern is Complex.

Reading clauses

Each optional clause ends at a comma, except IF, which runs to its , THEN marker. The clause keywords are matched case-insensitively. After the optional clauses, the remaining text up to shall is the system, which must start with the, and everything after shall is the response.