Regular expression ignore string if starts with specific substring

Question

I need to find with the regular expression domain names that don't start with the string "http". For example:

https://domain1.com -> Don't match
http://domain2.com -> Don't match
domain3.com -> Match
domain4.co.uk -> Match

I found a regex that almost got this:

(?:[a-zA-Z0-9](?:[a-zA-Z0-9\-]{,61}[a-zA-Z0-9])?\.)+[a-zA-Z]{2,6}

But it also detects "https://domain1.com"

Example given:

https://regex101.com/r/DjDBrx/1/

In this example I want to avoid "https://domain1.com"

Any help would be gratefully appreciated.

Are you validating full strings? Or extracting from longer texts? — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Aug 9, 2021 at 16:55
Welcome to SO! Do you want /^(?!http)/ or /\b(?!http)/? I don't understand the {,61} and {2,6}, and your pattern has no http in it, so it seems to have nothing to do with your written specification. — ggorlen
– ggorlen, Commented Aug 9, 2021 at 16:56
@ggorlen Thank you :) {,61} and {2,6} are for detecting domains. It has no "http" on it because I don't know how to add it in order to ignore "http" at the start. — Daniel
– Daniel, Commented Aug 9, 2021 at 17:13
I would use something like \b(?<!https:\/\/)(?:[a-zA-Z0-9](?:[a-zA-Z0-9-]{,61}[a-zA-Z0-9])?\.)+[a-zA-Z]{2,6}\b, a lookbehind with word boundaries. — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Aug 9, 2021 at 17:13

Wiktor Stribiżew · Accepted Answer · 2021-08-09 19:36:41Z

1

You can use a word boundary coupled with two negative lookbehinds:

\b(?<!http:\/\/)(?<!https:\/\/)(?:[a-zA-Z0-9](?:[a-zA-Z0-9-]{,61}[a-zA-Z0-9])?\.)+[a-zA-Z]{2,6}\b
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                                                ^^

The (?<!http:\/\/)(?<!https:\/\/) are two negative lookbehinds that will get triggered at the same location inside the string (since lookarounds are non-consuming patterns) and - after making sure the location is at the word boundary due to \b - they will fail the match if there is http:// or https:// immediately to the left of the current location.

answered Aug 9, 2021 at 19:36

Wiktor Stribiżew

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Comments

Alireza · Accepted Answer · 2021-08-09 17:05:03Z

0

You can use negative lookahead, I think it is usually the quickest option. It returns negative if contains the string you are excluding, like: ^(?!(http)).*

edited Aug 9, 2021 at 17:05

Alireza

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answered Aug 9, 2021 at 17:02

Themathix

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2 Comments

Daniel Over a year ago

How would be the full regex? Don't know where to add it.

Themathix Over a year ago

@Daniel The entire regex would be "^(?!(http:))". It will return everything that doesn't contain "http:"

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