CODEWITHSUNDEEP
regexregex-groupcapturing-group
Dugalle
Dugalle

Reputation: 33

Regex recursion captured string

I have a problem with a regex that has to capture a substring that it's already captured...

I have this regex:

(?<domain>\w+\.\w+)($|\/|\.)

And I want to capture every subdomain recursively. For example, in this string:

test1.test2.abc.def

This expression captures test1.test2 and abc.def but I need to capture: test1.test2 test2.abc abc.def

Do you know if there is any option to do this recursively?

Thanks!

Upvotes: 3

Views: 164

Answers (3)

Wiktor Stribiżew
Wiktor Stribiżew

Reputation: 627292

You may use a well-known technique to extract overlapping matches, but you can't rely on \b boundaries as they can match between a non-word / word char and word / non-word char. You need unambiguous word boundaries for left and right hand contexts.

Use

(?=(?<!\w)(?<domain>\w+\.\w+)(?!\w))

See the regex demo. Details:

  • (?= - a positive lookahead that enables testing each location in the string and capture the part of string to the right of it
    • (?<!\w) - a left-hand side word boundary
    • (?<domain>\w+\.\w+) - Group "domain": 1+ word chars, . and 1+ word chars
    • (?!\w) - a right-hand side word boundary
  • ) - end of the outer lookahead.

Another approach is to use dots as word delimiters. Then use

(?=(?<![^.])(?<domain>[^.]+\.[^.]+)(?![^.]))

See this regex demo. Adjust as you see fit.

Upvotes: 0

Schwern
Schwern

Reputation: 165386

You can use a positive look ahead to capture the next group.

/(\w+)\.(?=(\w+))/g

Demonstration.

Edit: JvdV's regex is more correct.

Note that \w+ is will fail to match domains like regex-tester.com and will match invalid regex_tester.com. [a-zA-Z0-9-]+ is closer to correct. See this answer for a complete regex.

It's simpler and more robust to do this by splitting on . and iterating through the pieces in pairs. For example, in Ruby...

"test1.test2.abc.def".split(".").each_cons(2) { |a|
  puts a.join(".")
}

test1.test2
test2.abc
abc.def

Upvotes: 1

JvdV
JvdV

Reputation: 75950

Maybe the following:

(\.|^)(?=(\w+\.\w+))

Go with capturing group 2

Upvotes: 3

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