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Baseline
isbn10("data")
takes an ISBN-13 format code as the data input and returns is in ISBN-10 format. Any dashes or other punctuation will be ignored. If the argument is not a valid ISBN code, the function returns an empty string. If such characters as in the source string and quotes are omitted, hyphen may be mis-parsed as minus signs giving a wrong result.
Consider a book with the ISBN-10 '1472268997' and the ISBN-13 '978-1472268990':
$MyString = isbn10("978-1472268990") gives the correct '1472268997' but note, without any hyphens.
but
$MyString = isbn10(978-1472268990) gives the incorrect '1472268012' but note, without any hyphens.