# a simplyfied email address check wrt. RFC2822
# i.e.
#	- display name is not supported
#	- obsolete specs are not honored
#	- [content] folding whitespaces are not allowed
# AND
#	- quoted-string is not allowed (and thus no control characters)
#	- domain-literal is not allowed
# so:
atext			=	ALPHA / DIGIT / "!" / "#" / "$" / "%" / "&" / "'" / "*" /
					"+" / "-" / "/" / "=" / "?" / "^" / "_" / "`" / "{" / "|"
					/ "}" / "~"
dot-atom-text	=	1*atext *("." 1*atext)
dot-atom		=	dot-atom-text
qtext			=	NO-WS-CTL / %d33 / %d35-91 / %d93-126
text			=	%d1-9 / %d11 / %d12 / %d14-127
quoted-pair		=	"\" text
qcontent		=	qtext / quoted-pair
quoted-string	=	DQUOTE *qcontent DQUOTE
local-part		=	dot-atom / quoted-string
NO-WS-CTL		=	%d1-8 / %d11 / %d12 / %d14-31 / %d127
dtext			=	NO-WS-CTL / %d33-90 / %d94-126
dcontent		=	dtext / quoted-pair
domain-literal	=	"[" *dcontent "]"
domain			=	dot-atom / domain-literal
addr-spec		=	local-part "@" domain

DQUOTE			=	%d34
text			=	%d1-9 / %d11 / %d12 / %d14-127


# gets reduced to
atext			=	ALPHA / DIGIT / "!" / "#" / "$" / "%" / "&" / "'" / "*" /
					"+" / "-" / "/" / "=" / "?" / "^" / "_" / "`" / "{" / "|"
					/ "}" / "~"
dot-atom-text	=	1*atext *("." 1*atext)
dot-atom		=	dot-atom-text
local-part		=	dot-atom
domain			=	dot-atom
addr-spec		=	local-part "@" domain	= dot-atom "@" dot-atom

# which yields e.g. to:
#Java:
static final String atext = "[\\p{Graph}&&[^\"(),.:;<>@\\[\\]]]";
static final Pattern dotatom =
	Pattern.compile("^" + atext + "+(?:\\." + atext + "+)*$");

public final String checkEmailAddr(String addr) {
	if (addr == null) {
		return "null address";
	}
	int idx = addr.indexOf('@');
	if (idx == -1) {
		return "invalid address";
	}
	String localPart = addr.substring(0,idx);
	Matcher matcher = dotatom.matcher(localPart);
	if (!matcher.matches()) {
		return "unsupported local-part '" + localPart + "'";
	}
	String domain =  addr.substring(idx+1);
	matcher.reset(domain);
	if (!matcher.matches()) {
		 return "unsupported domain '" + domain + "'";
	}
	return checkDomain(domain);
}

# RFC 1035 + 1101
let-dig		= ALPHA / DIGIT
let-dig-hyp	= let-dig / "-"
label		= let-dig *( *let-dig-hyp let-dig)
subdomain	= label *("." label)
domain		= subdomain / " "

static final Pattern domainLabelPattern =
	"^[\\p{Alpha}\\p{Digit}]+(?:-+[\\p{Alpha}\\p{Digit}]+)*$";

public final String checkDomain(String domain) {
	if (domain.length() > 255) {
		return "Domain too long";
	}
	String[] label = domain.split("\\.");
	// we require to have at least two labels and the last two ones must have
	// at least 2 characters
	int count = label.length-1;
	if (count < 1 || label[count].length() < 2 || label[count-1].length() < 2)
	{
		return "invalid domain";
	}
	Matcher m = domainLabelPattern.matcher("");
	for (count; count >= 0; count--) {
		if (label[count].length > 63) {
			return "label too long";
		}
		m.reset(label[count]);
		if (!m.matches()) {
			return "invalid domain";
		}
	}
	// everything else wrt. domain should be checked via CIDR
	return null;
}


OR Javascript:

function checkemail(str) {
	var idx = str.indexOf("@");
	if (idx == -1) {
		return false;
	}
	var localPart = str.substr(0,idx);
	// var atext = "[a-zA-Z0-9!#$%&'*+-/=?^_`{|}~]";
	// "^" + atext + "+(?:\." + atext + "+)*$"
	var dotatom = /^[-a-zA-Z0-9!#$%&'*+/=?^_`{|}~]+(?:\.[-a-zA-Z0-9!#$%&'*+/=?^_`{|}~]+)*$/ ;
	// even more simplified to avoid trouble with other languages
	// var dotatom = /^[-a-zA-Z0-9%&+=?_~]+(?:\.[-a-zA-Z0-9%&+=?_~]+)*$/ ;
    if (!dotatom.test(localPart)) {
    	return false;
    }
    var domain = str.substr(idx+1);
    if (!dotatom.test(domain)) {
    	return false;
    }
    if (domain.length > 255) {
    	return false; // domain too long
    }
    var label = domain.split("\.");
    var count = label.length-1;
    if (count < 1 || label[count].length < 2 || label[count-1].length < 2) {
    	return false; // top and 2nd level domain too short
    }
    var domainLabel = /^[a-zA-Z0-9]+(?:-+[a-zA-Z0-9]+)*$/;
    for (; count >= 0; count--) {
    	if (label[count].length > 63) {
    		return false; // subdomain too long
    	}
    	if (!domainLabel.test(label[count])) {
    		return false; // invalid characters
    	}
    }
    // even if numerical IPs are allowed, we choose to not accept them and avoid
    // CIDR check
    var tldIP = /[0-9]+$/ ;
    var allIP = /^[0-9]+(?:\.[0-9]+)*$/ ;
    if (tldIP.test(domain) || allIP.test(domain)) {
    	return false;
    }
    return true;
}