You change this bit:
ValidationExpression="^((A-Za-z0-9])*$"
To include the characters you want to allow.
At present, that doesn't work: the brackets aren't matched up as you have two '(', one ')' and one ']'. I suspect it should be:
ValidationExpression="^([A-Za-z0-9])*$"
Which allows upper- and lower-case letters, plus numeric digits only.
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Expresso[
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