Conditional Formatting doesn't support Regex directly. You would need to create a user-defined function (UDF) in VBA. For example:
Function FormatSalary(varField As Variant) As Boolean
FormatSalary = (varField) > 20000
End Function
This Function would use Regex before returning True or False. In the Conditional Formatting rule for the field, you would use Expression is
and enter:
FormatSalary([Salary])
notice that there is no equals-sign preceding this.
You can, I believe, do this programmatically when clicking a button, working with the FormatConditions
collection in VBA. However, I suspect that you might need to switch the form back and forth to Design View. (I haven't tried this recently.)