Perfect Email Validation Regular expression _ regular expression

The inter-domain name format is as follows:
Domain names are composed of specific character sets, English letters, numbers, and "-" (that is, hyphens or minus signs) of each country's text, but neither the beginning nor the end can contain "-", "-" and "-" cannot appear continuously. The letters in the domain name are not case-insensitive. The domain name can be up to 60 bytes (including suffix. com,. NET,. org, and so on).

/^[a-z] ([a-z0-9]*[-_]?[ a-z0-9]+) *@ ([a-z0-9]*[-_]?[ a-z0-9]+) +[\.] [A-z] {2,3} ([\.] [A-z] {2})? $/i;

/content/I forms a case-insensitive regular expression;
^ Match start
$ match End
[A-z] e-mail prefix must be an English letter beginning

([A-z0-9]*[-_]? [a-z0-9]+) * and _a_2, AAA11, _1_a_2 match, and A1_, Aaff_33a_, A__aa mismatch, if it is a null character, is also a match, * represents 0 or more.
* Represents 0 or more preceding characters.
[a-z0-9]* matches 0 or more English letters or numbers
[-_]? Match 0 or 1 "-" because "-" cannot appear continuously
[a-z0-9]+ matches 1 or more English letters or numbers, because "-" can not be done as the end

@ must have a @
([A-z0-9]*[-_]? [a-z0-9]+] + see above ([a-z0-9]*[-_]?[ a-z0-9]+) * explanation, but cannot be null, + represents one or more.
[\.] The special character (.) As a normal character
[A-z] {2,3} matches 2 to 3 English letters, typically COM or net.
([\.] [A-z] {2})? Match 0 or 1 [\.] [A-z] {2} (for example, CN, etc.) I don't know. com.cn the last part is not all two digits, if not please modify {2} to {starting words, end words}