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* [[Check digit]]. To detect transcription errors in which digits have been altered or transposed, the last digit of a pre-2007 ISBN must match the result of a mathematical formula incorporating the other 9 digits ([[International Standard Book Number#ISBN-10 check digits|ISBN-10 check digits]]).
==Validation
;Allowed character checks
:Checks to ascertain that only expected characters are present in a field. For example a numeric field may only allow the digits 0–9, the decimal point and perhaps a minus sign or commas. A text field such as a personal name might disallow characters used for [[Markup language|markup]]. An e-mail address might require at least one @ sign and various other structural details. [[Regular expression]]s can be effective ways to implement such checks.
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