In Java Regex - email validation, How to allow apostrophe before @ and not as first and last character before @ and not two consecutive apostrophe?

Regex Used:

EMAIL_VALID_REGEX = "^[_A-Za-z0-9-\\+]+(\\.[_A-Za-z0-9-]+)*@"+"[_A-Za-z0-9-]+(\\.[_A-Za-z0-9-]+)*(\\.[A-Za-z]{2,})$";

Now i need to allow apostrophe before @ and also it should not allow apostrophe as first and last character and also two consecutive apostrophe before @.

Can someone modify the above regex to handle the mentioned scenarios and can you explain it?

Valid emails: test@gmail.com, test's@gmail.com, t@gmail [Even if single character is passed before, it is valid] Invalid emails: 'test@gmail.com, test'@gmail.com, 'test'@gmail.com, test''s@gmail.com

You can replace (\\.[_A-Za-z0-9-]+)*@ with ([.'’][_A-Za-z0-9-]+)*@:

^[_A-Za-z0-9+-]+(?:[.'’][_A-Za-z0-9-]+)*@[_A-Za-z0-9-]+(?:\.[_A-Za-z0-9-]+)*\.[A-Za-z]{2,}$

Define in Java with

EMAIL_VALID_REGEX = "^[_A-Za-z0-9+-]+(?:[.'’][_A-Za-z0-9-]+)*@[_A-Za-z0-9-]+(?:\\.[_A-Za-z0-9-]+)*\\.[A-Za-z]{2,}$"

See the regex demo

The [.'’] part is a character class matching ., ' or ’.

The ^[_A-Za-z0-9+-]+(?:[.'’][_A-Za-z0-9-]+)*@ part now matches

• ^ - start of string
• [_A-Za-z0-9+-]+ - one or more underscores, hyphens, plus, letters or digits
• (?:[.'’][_A-Za-z0-9-]+)* - zero or more occurrences of
  • [.'’] - ., ' or ’
  • [_A-Za-z0-9+-]+ - one or more underscores, hyphens, plus, letters or digits
• @ - a @ char.

If there is a comma, or apostrophe, they will be allowed only in between the chars matched with the [_A-Za-z0-9-] character class, as both occurrences of [_A-Za-z0-9-] patterns on both ends of [.'’] is quantified with + (i.e. they require at least one char to match).

To apply the same restriction to -, _ and +, use

EMAIL_VALID_REGEX = "^[A-Za-z0-9]+(?:[+_.'’-][A-Za-z0-9]+)*@[_A-Za-z0-9-]+(?:\\.[_A-Za-z0-9-]+)*\\.[A-Za-z]{2,}$"

See this regex demo.