Regex: Excluding combinations of characters and limiting certain characters in Regex / jQuery (partly working)

Question

I am trying to validate an email input using Regex in jQuery - only for the following criteria:

Inputs I want to allow:

• lowercase letters (multiple)
• digits (multiple)
• dots / periods (multiple)
• dashes (multiple)
• @ sign (only one)

Inputs I want to prevent:

• dot / period, dash, @ sign at the beginning of the string
• dot / period, dash, @ sign at the end of the string
• two dots / periods OR two dashes in a row
• a dot / period and a dash (or a dash and a dot / period) in a row

So far I came up with the following but I am having trouble with excluding the combinations of two symbols and with limiting the @ sign to only one.

Can someone show me how to do this right ?

Note:
I am only looking for a basic validation for the above criteria and for English letters only (everything else I cover in jQuery).

My Regex:

var val = $(this).val();
var checkEmail = /(?!^[@.-])(?!.*@@)(?!.*\.\.)(?!.*--)(?!.*[@.-]$)[0-9a-z@\.\-]/g;

if(val.match(checkEmail) {
   console.log('input valid');
   } else {
   console.log('input invalid');
}

Many thanks in advance,
Tom

Answer 1

You can use the regex below to achieve your results:

^(?!.+?(?:\.-|\.\.|--|-@|\.@|@\.).+?)[^.-@ ][a-z0-9_.-]+@[a-z0-9_.-]+(?<=[^.-@ ])$

Explanation of the above regex:

^, $ - Represents start and end of test string resp.

(?!.+?(?:\.-|\.\.|--|-@|\.@|@\.).+?) - Represents negative lookahead not matching the test string if it contains consecutive .-, .., --, -@, .@ or @..

[^.-@ ] - Represents negative match if string starts with any of .-@.

[a-z0-9_.-]+@[a-z0-9_.-]+ - Represents the character set where the String contains anything in the characters mentioned above with only one @ sign.

(?<=[^.-@ ]) - Represents positive lookbehind which matches any test string not ending with .-@.

You can find the demo of above regex here.

IMPLEMENTATION IN JAVASCRIPT(JQUERY):

var val = "hello@gmail.com";
var checkEmail = /^(?!.+?(?:\.-|\.\.|--|-@|\.@|@\.).+?)[^.-@ ][a-z0-9_.-]+@[a-z0-9_.-]+(?<=[^.-@ ])$/gm;

if (val.match(checkEmail)) {
  console.log('input valid');
} else {
  console.log('input invalid');
}

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