Validate email address with multi level domains in PHP with RegEx

Question

Need to validate email address with single domain level or multi level domains.(actually domain level <=4)

Criteria:

• Email (username) should start with letter[a-z] and it may have numbers but not for 1st letter and periods(.) No other characters.
• After @ sign every domain should start with letter.(minimum length 2)
• Maximum domains <= 4 .

Ex: email@my.sub.mail.com

above example there 4 domains;

• .my
• .sub
• .mail
• .com

I try with this RegEx:

^[a-zA-Z](:?[a-zA-Z0-9._-])+(@[a-zA-Z]+[a-zA-Z0-9_-])+\.+(([a-zA-Z]){2,6})$

But above regex not validating multiple domains correctly.It's only get 1 domain. Ex: myemail@gmail.com

Online Regex : https://regex101.com/r/7SXS1Z/1

Answer 1

Maybe this is what you're looking for

^
[a-zA-Z][-.\w]*       # before @
@
[a-zA-Z][-.\w]+       # first subdomain
(?:
    \.[a-zA-Z][-.\w]+ # eventually others
){1,3}
$

See a demo on regex101.com.

Answer 2

Your regex applies much more rules in accepting an email address. E.g. allowing email addresses to include more than one @ symbol. Go as simple as your own rules:

^[a-z][^@]*@(([a-z][a-z0-9-]+)\.){0,3}(?2)$

Live demo