What RegularExpressionValidator.ValidationExpression should I use to allow only ICQ UIN like input?
xxx-xxx-xxx and xxx-xxx-xx and xx-xxx-xxx and xxxxxxxxx so on..
i.e. with dash as separator and without.
You can use the following simple expression.
^([0-9]-?){7,8}[0-9]$
The drawback is, that it allows things like 1-2-3-4-5-6-7-8. If you want to restrict the layout more, you can use complexer expressions.
^(?=([0-9]-?){8,9})([0-9]{2,3}-?)*(?<!-)$
This expression asserts that the string contains exactly eight or nine numbers and some dashes using the positive look ahead assertion (?=([0-9]-?){8}). Then it matches groups of two or three numbers optionaly separated by dashes and finally asserts that the string does not end with a dash using the negative look behind assertion (?<!-).
This still allows some irregular patterns like 12-34567-89. If you want to eliminate them, too, you will have to list all allowed patterns. But I suggest not to do so, but allow as much flexibility as possible - I would allow every string with eight or nine numbers and any number of dashes - even --123---4-5-67--8 and then reformt the user input in a predefined format.
^(-*[0-9]-*){8,9}$