Regular expression is a very powerful way to to validate values in the field.

To add regular expression rule to the field:

• double click on the field in the mail form;
• right click on the the rules tree (on the node that you want to add the rule to);
• select Add->Value Constraint;
  
• in the operator drop down select REGEXP and type the regular expression in the text box;
• press on the Y button. Now regular expression is added to the schema. Schema should be saved if you want this change to appear in the original file.