Regex: How to expand regex for domain name to include domains of type a.b.c.d?

Question

The following regex picks up most mail server names of the form mail.example.com

([a-zA-Z0-9-]+\.{1,}[a-zA-Z0-9-]+\.[a-zA-Z0-9-]{2,})

as shown here.

How do we expand it such that it matches domains with one (or more) additional subdomains e.g.

b-app05-06.boldchat.com
ns126a.ba1.enops.net
NHQSDFEXCHUB01.nam.coair.com
ncsmcexchub01.nam.coair.com

Answer 1

You can enclose the last subpattern into a non-capturing group and set a + quantifier:

\b[\w-]+(?:\.[\w-]+){2,}\b

EXPLANATION:

• \b - Word boundary
• [\w-]+ - A character class that matches an alphanumeric or a hyphen
• (?:\.[\w-]+){2,} - a non-capturing group that matches 2 or more sequences of a literal dot and 1 or more alphanumeric characters or hyphen
• \b - Word boundary

See demo

Answer 2

you can take advantage of regex recursion if your engine support it (PCRE compatible one usually) Demo here

This regex would work for any number of subdomains and allow you to capture the inner domain.

(([\w-]+)[.](\w{2,}$|(?1))) in details:

• (([\w-]+)[.] start the whole capture for recursion, then start the capture for the leftmost subdomain followed by a dot.
• (\w{2,}$|(?1))) Alternation, try to match the tdl (end of match) or repeat the pattern.

You have the host in second capture group and it's domain in the third capture group. and the whole match in the first capture group (see the substitution pane in the demo)

Answer 3

You can do it shorter:

((?:[a-zA-Z0-9-]+\.)+[a-zA-Z0-9-]{2,})$

Demo

Answer 4

First, here is how to match one domain (given your examples):

[a-zA-Z0-9]+(?:-[a-zA-Z0-9]+)*

A tld gets matched with:

[a-z]{2,}

Now, you can have several domains separated by dots, followed by a tld:

((?:[a-zA-Z0-9]+(?:-[a-zA-Z0-9]+)*\.)+[a-z]{2,})

Debuggex Demo

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If you need to match domains composed by at least 2 subdomains + tld:

((?:[a-zA-Z0-9]+(?:-[a-zA-Z0-9]+)*\.){2,}[a-z]{2,})

Debuggex Demo