Validating inputs prevents the user from accidentally or intentionally misusing a program.

Table 1 shows the five main types of validation checks.

Table 1

Check	Description
Range check	Checks whether the input (usually numerical or date) is within a given range
Type check	Checks whether the input is of the correct data type (integer, string, real, etc.)
Length check	Checks the length of the input (e.g. longer than 8 characters)
Presence check	Checks that the input is present (i.e. has not been left blank)
Format check	Checks the format of the input (e.g. email address must contain '@')

Authentication is the process of checking that a particular user is permitted to use a system and, if so, which parts of that system they can use.

(i.e., authentication makes sure that a person is actually who they say they are)

A common example of an authentication measure is the username/password combination.

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A website wants to only accept usernames which are 5 to 10 characters and start with a letter. Which two validation checks would they need to use?

Length check to make sure it's 5 to 10 characters.
Format check to make sure it starts with a letter.

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