How to not check a textbox if empy

Hello,

I need to check the format of a string in a textbox.
But I want to check ONLY if there is something in the textbox.
I need to have the possibility to not enter a value in this textbox.

Here is the pseudo-code to check the format:

<!-- CHECK FOR VALID FORMAT -->
<sch:assert test="
...... check format......
   ">
   <sch:name/> does not contain a valid format.
</sch:assert>

How could I do that?

Thanks
Sylvain


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