You can use the custom validator to achieve this. I recommend CustomValisator over RegularExpressionValidator for the reason that you also need the fields as required. This thing can be easily achieved using CustomValidator control
To get stared with Custom Validator control refer
1.
Custom Validation Summary in ASP.NET[
^]
2.
http://asp.net-tutorials.com/validation/custom-validator/[
^]
Once you use the CustomValidatorControl, you can put the regular expression in the javascript code to validate the data in the textbox
1. Regular Expression for Number only :
[0-9]*
2. Regular Expression for Characters only :
[A-Za-z]*
3. Regular Expression for Email :
\b[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}\b
Hope this helps.
All the best