RegEx pattern for alphanumeric or numeric entry

Hello, great plugin.
However, i can’t figure out a RegEx pattern for a GF form field that may not accept letters only (it’s a passport number field). It must have at least one number digit, it can also have all numbers. But it can’t have all letters.

Examples:
X12345678 – valid
ABCDEFGH4 – valid
123456789 – valid
ABCDEFGHJ – invalid -> validation error message

Can anybody help?

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