How to check if two fields contain same value

All you need to do is add three fields (two for e-mails, one for the validation message):
E-mail1

• Column name: email1
• Attribute type: Text
• Attribute size: 100
• Allow empty values: false
• Form control: E-mail
• Has depending fields: true

E-mail2

• Column name: email2
• Attribute type: Text
• Attribute size: 100
• Allow empty values: false
• Form control: E-mail
• Has depending fields: true

 Validation message

• Column name: compare
• Attribute type: Text
• Attribute size: 100
• Allow empty values: true
• Form control: Label
• Regular expression: .+
• Error message: E-mails have to match
• Visible condition: email1.Value!=email2.Value
• Depends on another field: true

And create proper form layout to hide the compare field caption, like:
$$label:EmailID$$    $$input:EmailID$$    $$validation:EmailID$$
$$label:email1$$    $$input:email1$$    $$validation:email1$$
$$label:email2$$    $$input:email2$$    $$validation:email2$$$$input:compare$$$$validation:compare$$