Using a regular expression to validate an zip code

A string must contain the following

1. 123， => Three digits with Comma
2. 123 => Three digits with Nothing
3. ，123 => Comma with Three digits
4. 1234567， => Seven digits with Comma
5. 1234567 => Seven digits with Nothing
6. ，1234567 => Comma with Seven digits

Maybe you don't understand what I want, so let me give you an example

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123-------------------------right
1234------------------------wrong
123，1234----------------wrong
,123-------------------------right
,123,------------------------right
123,123--------------------right
123,123,-------------------right
123,1234567-------------right

The three digits can be replaced by a number of seven digits