An example beats an abstract rule
Aliases: format example · worked example · abstract pattern copy
What it is
“8–20 letters, digits, and underscores” is an abstract rule; “e.g. user_01” is an example. When filling a formatted field, people more often imitate a visible instance than compile the sentence into a mental regex. Example-over-rule says: when shape must be conveyed, lead with a legal instance and let the rule supplement; an abstract description alone produces warped copies. It does not care whether the hint appears before typing or after an error, nor whether it stays on screen.
Why it happens
An abstract rule needs a translation from language into shape: length, character class, and separator positions are encoded as prose. Translation drops detail—“alphanumeric” may or may not include spaces; “date” may be day-month-year or year-month-day; the sentence is often vague. An example lays the shape out: how many groups, which positions are digits, where the marks sit. That is the same path as worked-example learning: imitate a concrete object, then generalize. A bad example is equally strong: a phone that looks real will be submitted; a single regional format trains other legal variants as errors. Examples are strong constraints, so they must be chosen.
Studying it
Present the same constraint as an abstract sentence, a single example, or example-plus-one-line rule. Compare first-submit legality, self-reports of legal variants being rejected, and whether the example string itself is submitted. Factors: whether several legal writings exist, whether the example is obviously fake. Outcomes: imitation accuracy, example-as-value, over-narrowing (only strings isomorphic to the example accepted). Do not replace the typed string with a “did you understand the rule?” questionnaire—people will say yes and still follow the example’s spaces.
Where it stops holding
An example cannot cover mutually exclusive legal shapes (international numbers with or without a country code, multiple local dates); one instance trains the rest as wrong. Security copy such as “don’t use common words” is hard to exemplify without publishing a dictionary of weak passwords. A regex is right in developer docs and worse than Chinese prose for end users. When character class must be exact (check digits, VINs), the example needs a still-visible length; a lone instance is read as “length may change.”
Applying it
- Show shape with one obviously unreal legal instance, plus one sentence that only adds what the example left unsaid.
- When several writings are legal, either show two contrasting instances or say “one of the following”; do not let a single example become the only template.
- Do not make the example look like a submittable true value (a real mobile number, a real name).
- Verify by hiding the abstract sentence and leaving the example: is the typed string legal? Then offer a legal value that is not isomorphic to the example and see whether people refuse to submit. Fail the first, change the example; fail the second, add variant copy.