Implement Custom Field Validation

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Hi,

I want to implement a validation that checks (per keystroke) if the value of a field is according to requirements. If I use a page level variable in JET that holds the field value, it is only updated when the user leaves the field. I need the validation to fire on every keystroke. 

You can think of something like a password validator like explained in the JET CookBook: https://www.oracle.com/webfolder/technetwork/jet/jetCookbook.html?component=passwordInput&demo=patternMatching

What is the best way of implementing this in a VBCS application. An example would be great,