Question

I want to know how this regular expression is expand and how it validates proper E-mail address ?

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

Answer 1

What is a valid email address? It’s usually name@FQDN

Whereby FQDN is “fully qualified domain name”
A FQDN must consist of one hostname and one top-level domain.
It CAN have one or more optional subdomains.

^ start of match (see below)

[_a-z0-9-]+ the very first character of a valid email account name (required, hence the + qualifier 1..n)

(\.[_a-z0-9-]+)* optional characters for a valid email account name (hence the * quantifier 0..n)

@ @-literal (delimiting account name from FQDN)

[a-z0-9-]+ character set for domain names (second-level domains / subdomain)

(\.[a-z0-9-]+)* character set for domain names (second-level domains / subdomain)

(\.[a-z]{2,3}) character set for top level domains (.com, .net, etc. – won’t be useful for the ‘new’ TLDs like .info, .business, .museum and so on

$ end of match

That ^ . . . $ is often used to declare that the whole string must consist of the pattern (and not just include it somewhere).