Issue

• The issue occurs on the Groups Management tab of the "Users and Groups Management" portlet when adding a user to a group. In one case the username is composed of numeric characters. When clicking Save on the screen, a validation warning dialog appears with the following message:

  'The field "User Name" can only contain alpha, digit, comma, dash characters. 
  The first and last character must be a letter.'
  

Further investigation shows that hard coded regular expressions are used for validations. For example, the following code is in the org.exoplatform.organization.webui.component.UIGroupMembershipForm class:

  addUIFormInput(new UIFormStringInput(USER_NAME, USER_NAME, null)
                     .addValidator(MandatoryValidator.class)
                     .addValidator(ExpressionValidator.class, 
                        "^\\p{L}[\\p{L}\\d._\\-,]+$", "UIGroupMembershipForm.msg.Invalid-char"
                     )
                );

Is it possibly to make the validation expression configurable?