>I have a character 8 field, which should always have 'A0' as two first letters and all other 6 letters must be digiits. I'm thinking, how should I implement such control...
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>Do you have ideas?

Since the first two characters are always AO, don't (since it is pointless) store those in the table, instead just display it as a label in front of the field. The field will then be a 6 digit numeric field. You can then have an input mask of 999999 and a maxlength of 6.