RegEx for failing subdomains

Question

Basically, I would like to check a valid URL that does not have subdomain on it. I can't seem to figure out the correct regex for it.

Example of URLs that SHOULD match:

• example.com
• www.example.com
• example.co.uk
• example.com/page
• example.com?key=value

Example of URLs that SHOULD NOT match:

• test.example.com
• sub.test.example.com

Answer 1

Here, we would start with an expression which is bounded on the right with .com or .co.uk and others, if desired, then we would swipe to left to collect all non-dot chars, add an optional www and https, then we would add a start char ^ which would fail all subdomains:

^(https?:\/\/)?(www\.)?([^.]+)(\.com|\.co\.uk)(.+|)$

---

Other TLDs can be added to this capturing group:

(\.com|\.co\.uk|\.net|\.org|\.business|\.edu|\.careers|\.coffee|\.college)

And the expression can be modified to:

^(https?:\/\/)?(www\.)?([^.]+)(\.com|\.co\.uk|\.net|\.org|\.business|\.edu|\.careers|\.coffee|\.college)(.+|)$

---

Flexibility

I can't think of something to make the TLDs too flexible, since this is a validation expression. For instance, if we would simplify it to:

^(https?:\/\/)?(www\.)?([^.]+)(\.[a-z]+)(\.uk?)?[a-z?=\/]+$

it might work for the URLs listed in the question, but it would also pass:

example.example

which is invalid. We can only use this expression:

^(https?:\/\/)?(www\.)?([^.]+)(\.[a-z]+)(\.uk?)?[a-z?=\/]+$

if we would know that what we pass, it is already a URL.

NOT FUNCTIONAL DEMO

Demo

This snippet just shows that how the capturing groups work:

const regex = /^(https?:\/\/)?(www\.)?([^.]+)(\.com|\.co\.uk)(.+|)$/gm;
const str = `example.com
www.example.com
example.co.uk
example.com/page
example.com?key=value

test.example.com
sub.test.example.com`;
let m;

while ((m = regex.exec(str)) !== null) {
    // This is necessary to avoid infinite loops with zero-width matches
    if (m.index === regex.lastIndex) {
        regex.lastIndex++;
    }
    
    // The result can be accessed through the `m`-variable.
    m.forEach((match, groupIndex) => {
        console.log(`Found match, group ${groupIndex}: ${match}`);
    });
}

RegEx Circuit

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RegEx

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DEMO