Validate text with javascript RegEX

Question

I'm trying to validate text with javascript but can find out why it's not working.

I have been using : https://regex101.com/ for testing where it works but in my script it fails

    var check = "test"
    var pattern = new RegExp('^(?!\.)[a-zA-Z0-9._-]+$(?<!\.)','gmi');
    if (!pattern.test(check)) validate_check = false;else validate_check = true;

What i'm looking for is first and last char not a dot, and string may contain [a-zA-Z0-9._-]

But the above check always fails even on the word : test

Ephellon Grey · Accepted Answer · 2021-10-24 18:53:21Z

1

+$(?<!\.) is invalid in your RegEx

$ will match the end of the text or line (with the m flag)
Negative lookbehind → (?<!Y)X will match X, but only if Y is not before it

answered Oct 24, 2021 at 18:53

Ephellon Grey

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3 Comments

Ephellon Grey Over a year ago

Also, the [a-zA-Z0-9._-] is invalid; try using [\w\.\-]

Reflective Over a year ago

It's not invalid? What makes you think it is? Well, not optimal, but not invalid!

Reflective Over a year ago

only . should be escaped probably i.e \.

Reflective · Accepted Answer · 2021-10-24 19:41:18Z

0

What about more simpler RegEx?

var checks = ["test", "1-t.e_s.t0", ".test", "test.", ".test."];
checks.forEach(check => {
  var pattern = new RegExp('^[^.][a-zA-Z0-9\._-]+[^.]$','gmi');
  console.log(check, pattern.test(check))
});

Your code should look like this:

var check = "test";
var pattern = new RegExp('^[^.][a-zA-Z0-9\._-]+[^.]$','gmi');
var validate_check = pattern.test(check);
console.log(validate_check);

edited Oct 24, 2021 at 19:41

answered Oct 24, 2021 at 19:04

Reflective

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Comments

The fourth bird · Accepted Answer · 2021-11-11 22:30:41Z

A few notes about the pattern:

You are using the RegExp constructor, where you have to double escape the backslash. In this case with a single backslash, the pattern is ^(?!.)[a-zA-Z0-9._-]+$(?<!.) and the first negative lookahead will make the pattern fail if there is a character other than a newline to the right, that is why it does not match test
If you use the /i flag for a case insensitive match, you can shorten [A-Za-z] to just one of the ranges like [a-z] or use \w to match a word character like in your character class
This part (?<!\.) using a negative lookbehind is not invalid in your pattern, but is is not always supported

For your requirements, you don't have to use lookarounds. If you also want to allow a single char, you can use:

^[\w-]+(?:[\w.-]*[\w-])?$

^ Start of string
[\w-]+ Match 1+ occurrences of a word character or -
(?: Non capture group
- [\w.-]*[\w-] Match optional word chars, a dot or hyphen
)? Close non capture group and make it optional
$ End of string

Regex demo

const regex = /^[\w-]+(?:[\w.-]*[\w-])?$/;
["test", "abc....abc", "a", ".test", "test."]
.forEach((s) =>
  console.log(`${s} --> ${regex.test(s)}`)
);

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