How to exclude a specific literal in Pattern

Question

I have a <input type="email"/> and want it to
a) be a valid email address and
b) be something other than a given email address - let's say "root@localhost".

I had a look into the Pattern for HTML input elements and now I try to write a pattern excludes a specific value.

I stumbled across this Post: A regular expression to exclude a word/string and looked into negative look ahead, which was quite promising. Except for one thing:
If I use negative lookahead, it will also match anything that starts with the given exlusion, not only those that exactly match it!

My Code:

<input type="email" name="email" pattern="^(?!root@localhost).+$" required>

This will result in a validation error for

empty text (correct)
non-email-texts (correct)
an email-address of "root@localhost" (correct)
any email addresss starting with "root@localhost" (wrong)

Can you please help me on how to write a pattern that excludes exactly one email address from the validation?

Is using a pattern the right way here?

Thanks,
Alexander

Use `pattern="^(?!root@localhost$).*$"` or just `pattern="(?!root@localhost$).*"` since it is anchored by default. — Wiktor Stribiżew, Nov 14 '18 at 16:30
@WiktorStribiżew looks like you're right. `pattern="(?!root@localhost$).*` seems to work. Feel free to post it as an answer and I'll accept it — Alexander, Nov 14 '18 at 16:36

score 1 · Accepted Answer · answered Nov 14 '18 at 16:53

1

You may use

pattern="(?!root@localhost$).*"

Since the HTML5 patterns are anchored at both ends automatically you don't have to enclose the pattern with ^ and $.

The overall regex after HTML5 engine processes the attribute value will look like /^(?:(?!root@localhost$).*)$/ (in FF and Chrome, it will be compiled with u flag) and will match any 0 or more chars other than line break chars, from the start till the end of a string, that is not equal to root@localhost string.

answered Nov 14 '18 at 16:53

Wiktor Stribiżew

607,720
39
448
563

How would I exclude multiple different values? For example I'd like to exclude `root@localhost` *and* `root@gmail`. – Dominik Krzywiecki Dec 16 '22 at 13:16
@DominikKrzywiecki `pattern="(?!root@(?:localhost|gmail)$).*"` – Wiktor Stribiżew Dec 16 '22 at 13:17

score 0 · Answer 2 · answered Nov 14 '18 at 16:35

0

Why not use javascript validation

function validate(form) {
    if (form.email === "root@localhost") {
        return false;
    }

    return /.+\@.+\..+/.test(form.email);
}

answered Nov 14 '18 at 16:35

Nikhil Ranjan

994
12
16

there are reasons for that. Pattern is an elegant way to perform a validation on any input field where no piece of Javascript needs to be written. – Alexander Nov 14 '18 at 16:36

How to exclude a specific literal in Pattern

2 Answers2