--I don't know the full rules, but here's some simple SQL code

--which does at least some of it.

--It could be used in a where clause or case statement,

--inside a function or not.

--More conditions can easily be added along similar lines,

--so this ought to be able to do what you want with some tweaking.

--

--This allows any number of dots, and one @.

--One of the dots must be after the @.

--

--For all dots, and the @:

--none of these may be at the beginning or end of the string,

--and no two may be adjacent.

--

--All other characters must be alphanumeric or _ or -

 

@vc table(vc varchar(50))

@vc

'dhskjdhfkss' union all

'dhsk@fkss' union all

select ';dhsk@fkss.ooo' union all

'dh-sk@fk-ss.ooo' union all

'dh\sk@fk-ss.ooo' union all

'@fkss.das' union all

'dhsk@fkss.a.asd.asd.aaaaaa' union all

'dhsk@fkss.ddd..dddd' union all

'dh+k@fkss.ddd.d' union all

'dhsk@fkss.ddd.kk.dd' union all

'dhsk@f#ss.ddd' union all

'd--k@f_ss.ddd' union all

'------@__-ss.___._._._' union all

'dhsk@f#ss.ddd' union all

'dh_k@fkss.ddd' union all

'dhsk@fkss.' union all

'.dhsk@fkss.nnn' union all

'sgsgaha.sksjkajd.d.d.dhsk@fkss.nnn.dsdsd.as' union all

'd.h.s.k@f.k.s.s.f' union all

'd@f.s' union all

'd.hs@fkss.d' union all

'd.sk.@ggg.fkss.ddd'

vc.vc VALID____________________________

@vc vc

not vc.vc like '%[^a-Z0-9.@!_-]%' escape '!' --contains only alphanumeric, '-', '.' and '@'

not vc.vc like '%@%@%' --has no more than one '@' sign

not vc.vc like '%..%' --no adjacent dots

not vc.vc like '.%' --no leading dot

and vc.vc like '%[^.]@[^.]%._%' --has an '@' without dots next to it, and to its right a dot followed by another char.

vc.vc INVALID__________________________

@vc vc

vc.vc like '%[^a-Z0-9.@!_-]%' escape '!' --contains only alphanumeric, '-', '.' and '@'

vc.vc like '%@%@%' --has no more than one '@' sign

vc.vc like '%..%' --no adjacent dots)

vc.vc like '.%' --no leading dot

not vc.vc like '%[^.]@[^.]%._%' --has an '@' without dots next to it, and a dot afterwards.