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into two four-digit numbers. The last digit, which may be zero through nine or an X, is a check digit, so the ISSN is uniquely represented by its first seven digits. Formally, the general form of the ISSN (also named "ISSN structure" or "ISSN syntax") can be expressed as follows:where N is in the set {0,1,2,...,9}, a decimal digit character, and C is in {0,1,2,...,9,X}; or by a Perl Compatible Regular Expressions (PCRE)

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