Question

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Regexp recognition of email address hard?

Hi,

I would like to implement validator for only local part of the email address.

Any suggestions please welcome.

Answer 1

From the Email address article at wikipedia (Syntax section):

The local-part of the email address may use any of these ASCII characters:

• Uppercase and lowercase English letters (a–z, A–Z)
• Digits 0 to 9
• Characters ! # $ % & ‘ * + – / = ? ^ _ ` { | } ~
• Character . (dot, period, full stop) provided that it is not the first or last character, and provided also that it does not appear two or more times consecutively (e.g. [e-mail]).

The syntax is formally defined in RFC 5322 section 3.4.1 and RFC 5321. It is defined by a grammar in which the local part starts like this:

local-part      =   dot-atom / quoted-string / obs-local-part
atext           =   ALPHA / DIGIT /    ; Printable US-ASCII
                   "!" / "#" /        ;  characters not including
                   "$" / "%" /        ;  specials.  Used for atoms.
                   "&" / "'" /
                   "*" / "+" /
                   "-" / "/" /
                   "=" / "?" /
                   "^" / "_" /
                   "`" / "{" /
                   "|" / "}" /
                   "~"

atom            =   [CFWS] 1*atext [CFWS]

dot-atom-text   =   1*atext *("." 1*atext)

dot-atom        =   [CFWS] dot-atom-text [CFWS]

...