{"id":62,"date":"2015-11-26T05:21:58","date_gmt":"2015-11-26T13:21:58","guid":{"rendered":"https:\/\/cloudinsidr.com\/content\/?p=62"},"modified":"2015-11-26T16:29:17","modified_gmt":"2015-11-27T00:29:17","slug":"tip-of-the-day-how-to-extract-domain-names-from-email-addresses-using-regular-expressions-regex","status":"publish","type":"post","link":"https:\/\/www.cloudinsidr.com\/content\/tip-of-the-day-how-to-extract-domain-names-from-email-addresses-using-regular-expressions-regex\/","title":{"rendered":"Tip of the Day: How to Extract Domain Names from Email Addresses using Regular Expressions (RegEX)"},"content":{"rendered":"

Extracting domain names from email addresses with the help of\u00a0regular expressions takes just a nanosecond\u00a0once you have the formula. The formula is the key.<\/p>\n

<\/p>\n

This is how you let your text editor find (and “understand”) each email address as two groups of characters separated by the @ symbol:<\/p>\n

^(\\S+)@(\\S+)<\/pre>\n
This is how you perform the replacement, removing everything except for the domain name (as defined by the second group of characters in the regex\u00a0above):<\/p>\n
($2)<\/pre>\n
It’s that easy.<\/p>\n
\"RegEx:<\/a>
A regular expression to extract domain or host names from email addresses (here in Notepad++)<\/figcaption><\/figure>\n
Practical Use Cases<\/h3>\n
One notable use case: Your email client may have collected untold amounts of email addresses from spammers. Now instead of blocking them on the client level (after delivery to your sorry inbox), you may want to block them at the mail server level. That’s easy enough (for example, you could be using smtpd sender restrictions in Postfix<\/a>), but what if you wanted to block all<\/em> senders from the offending domains? Exactly. You need to remove the user names in front of the @ sign, leaving only\u00a0the domain names in the file. That’s where a regular expression\u00a0can come in handy.<\/p>\n
One more\u00a0thing: how do you come up with\u00a0a regular expression of your own? By using\u00a0a really good RegEx editor like regex101.com<\/a> and testing your regular expression\u00a0on a sample of data.<\/p>\n
Assuming that you want to match any host name on each of the domains in your Postfix configuration and reject each\u00a0message that matches the sender’s address, you need to prefix each of the domain names\u00a0in your list with a dot, append a space and the REJECT directive.<\/p>\n
\"RegEx<\/a>
A regular expression to extract domain or host names from email addresses for smtpd client restrictions in Postfix: spam originating from those domains will be rejected<\/figcaption><\/figure>\n
Once this is done, you may end up with a list full of duplicates. You can sort it and remove the duplicates in Notepad++ using the module TextFX and\u00a0the following commands:<\/p>\n