Seven validation checks, and when to use each

By the end of this lesson

By the end you can name all seven validation checks, say exactly what each one tests, and pick sensible checks for a given field.

Each check tests one property

A validation check is a rule about what values a field may hold. Each rule tests one property of the value — is it there, is it the right length, is it in the right pattern, is it in the allowed set, and so on.

The trick to choosing a check is to finish this sentence: "this field can only hold a value that is …". The end of the sentence tells you which check to use.

Check	The question it asks	Example field	Value that fails
Presence	Is it there at all?	Surname	blank
Length	Is it the right number of characters?	Phone number	8 digits when 11 expected
Range	Is it between two limits?	Exam mark	105
Limit	Is it on the allowed side of one limit?	Hours worked	−2, or 45 when the rule is "no more than 40"
Format	Does it match the required pattern?	Date	`31/13/2026`
Existence	Is it in the list of real values?	Customer ID	`C9999` when no such customer exists
Check digit	Do the digits agree with the calculated check?	Barcode	one digit changed

Definition

The seven validation checks

Presence check — the field must contain something; a blank is rejected.
Length check — the value must have a stated number of characters (or at least, or at most, that number).
Range check — the value must lie between a lower and an upper limit, inclusive.
Limit check — the value must satisfy a single boundary, such as "no more than 40" or "zero or more".
Format check — the value must match a pattern, such as two digits, a slash, two digits, a slash, four digits for a date.
Existence check — the value must appear in a stored list of valid values (a reference table).
Check digit — a digit calculated from the others is recalculated and compared, so a mistyped digit is detected.

The two that get confused

Range and limit look similar but are not the same.

A range check has two boundaries: a mark must be 0 to 100 inclusive.
A limit check has one: an order quantity must be greater than 0, with no upper boundary at all.

So "the temperature must be between −10 and 40 °C" is a range check. "The overdraft must not exceed £500" is a limit check, because only the top is bounded.

Length and format are also easy to mix up.

Length counts characters: a UK mobile number must be 11 digits.
Format tests the pattern: a date must be two digits, slash, two digits, slash, four digits.

A value can satisfy one and fail the other: 12345ABCDE6 is 11 characters (length passes) but is not all digits, so the format check fails.

Combining checks on one field

Real fields usually need more than one rule, applied in a sensible order.

Take a date of birth on a registration form:

Presence — something must be entered.
Format — it must be DD/MM/YYYY.
Range — the year must be between 1900 and the current year.
A second range check — the month must be 1 to 12, the day 1 to 31.

The order matters for the user experience: checking presence first avoids a confusing "invalid format" message for an empty box.

What no combination of these rules can do is confirm that the date belongs to the person filling in the form. Validation checks structure; only verification compares with a source.

Name that check

Multiple choiceQuestion 1 · Recall

Fill in the blanksQuestion 2 · Apply

Multiple choiceQuestion 3 · Apply

Answer the 3 questions above to continue.