Validate pattern OR nothing in XSD

Say you have a XML like this:


Where <date> is allowed to be empty OR follow pattern ’19|20[0-9]{2}[0-1][0-9][0-3][0-9]’. How do you validate it? The solution is too simple and that is why I didn’t figure it out myself. I googled it and soon found a much smarter guy than me that said to do it like this:


            <xs:pattern value="(19|20[0-9]{2}[0-1][0-9][0-3][0-9])|"/>


This validates YYYYMMDD type dates and also validate an empty date tag. The solution lies in the last OR operand ‘|’. You see, either the left one (the pattern) is true or the right one (which is empty). Cleaver ah? I certainly thinks soo

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