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With this simplest RegEx-filter I'd like to catch images called ad1.gif or ad4.png. This works but I wonder why it doesn't match when the filename doesn't contain the number: ad.gif slips through.

Why that? I thought the question mark means "one or zero"?

Yes it does but it works on the preceding character which in this case is 'd' so 'a1.gif' or 'a4.gif' currently match as well. The filter should really be:

Ah, I see. Thanks for the help.