Pensieve Header: Validating IBAN numbers as in http://en.wikipedia.org/wiki/International_Bank_Account_Number#Algorithms

iban="GB82WEST12345698765432"

StringLength[iban]

RotateLeft[Characters[iban], 4]

ToCharacterCode["09aA"]

Flatten[
ToCharacterCode /@ RotateLeft[Characters[iban], 4]
] /. {
c_Integer /; 48≤c≤57 :> ToString[c-48],
c_Integer /; c≥65 :> ToString[c-55]
}

StringJoin[Flatten[
ToCharacterCode /@ RotateLeft[Characters[iban], 4]
] /. {
c_Integer /; 48≤c≤57 :> ToString[c-48],
c_Integer /; c≥65 :> ToString[c-55]
}
]

mod97[iban_String]:=Mod[ToExpression[StringJoin[Flatten[
ToCharacterCode /@ RotateLeft[Characters[iban], 4]
] /. {
c_Integer /; 48≤c≤57 :> ToString[c-48],
c_Integer /; c≥65 :> ToString[c-55]
}
]], 97];
mod97[iban]

mod97["IL000204470000000100664"]

mod97["IL310204470000000100664"]

StringLength["IL830204470000000100664"]

mod97["GB82WEST12345698765432"]