There are a few approaches you can take here, which I will list in order of preference. These should work just about anywhere merge fields can be used.

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**Create a Custom Permission**

You can check if someone has a `Custom Permission` in a formula as follows:

    $Permission.My_Custom_Permission__c

Steps:

1. Create `Custom Permission`
2. Add it to a `Permission Set`
3. Assign that `Permission Set` to any `User` you wish to exempt from validation

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**Hierarchy Custom Setting**

If you don't find the above palatable for some reason, you can also check a `Hierarchy Custom Setting` in a formula as follows:

    $My_Hierarchy_Setting__c.Is_Exempt_From_Validation_X__c

Steps:

1. Create `Hierarchy Custom Setting`
2. Add exemption field
3. Set `Org Wide Default` for this field to false
4. Set value to true for any `User` you wish to exempt from validation

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**Username**

You can check against a `Username` in a way that at least works in all environments as follows:

    BEGINS($User.Username, "[email protected]")

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**User Id**

I would not use this unless you have compelling reasons none of the above are a good fit:

    $User.Id = "00550000000lxVg"