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Asked: 2026-05-26T02:02:52+00:00

I’m writing a regular expression for Postfix that defines its virtual domains map. I

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I’m writing a regular expression for Postfix that defines its virtual domains map. I want to catch all subdomains of a domain except for two.

Assuming my domain is example.com and the two exclusion subdomains are in,mail, I wrote the following regular expression:

(?(?!mail|in).+\.example.com)

It supposed to recognize whatever.example.com but not in.example.com or mail.example.com.

It does work in RegExr, but it doesn’t work in Postfix nor Ruby. I’m assuming that I’m using if-then wrong, what is the correct syntax? Are there other options?

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1 Answer

  1. Editorial Team

    If you use an anchor, then it should work.

    \b(?!mail|in).+\.example.com

    See it here on Rubular

    The anchor at the beginning \b is a word boundary. This assures that there is non word character before your subdomain and therefor it does not match ail from mail anymore.

    This matches

    http://www.example.com
    whatever.example.com

    and not

    mail.example.com
    in.example.com

    UPDATE

    Probably the ^ anchor is the better choice here. This would match the start of the string.

    ^(?!mail|in).+\.example.com

    For your examples it makes no difference, but if your URL starts with a non-word character then it would be wrong with the word boundary.

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