It worked!!
dop2000 I followed your script and it's exactly what I needed. Thank you very much
In addition I started to investigate according to your advice on len () and I found how to put a maximum of characters, in case someone needs it, this is the result.
System Compare two values
RegexSearch(TextBox.Text, "[a-zA-Z]{3}_[a-zA-Z0-9]{6}","g")=0
-> Button set enabled
maximum number of characters: 6
System Compare two values
len(TextBox.Text)>=7
-> TextBox set text left(TextBox.Text, 6)