Following is the regex I am trying to use
$eregicheck = "^([-!#\$%&'*+./0-9=?A-Z^_`a-z{|}~])+@([-!#\$%&'*+/0-9=?A-Z^_`a-z{|}~]+\\.)+[a-zA-Z]{2,4}\$";
I changed the following lines
return eregi($eregicheck, $emailtocheck);
to
return preg_match($eregicheck, $emailtocheck);
But I dont know why I am getting the error
Warning: preg_match() [function.preg-match]: Unknown modifier '_'
You're getting this error because php requires delimiters before and after the regex pattern, which in your case it assumes to be ^ and after the delimiters follow the modifiers, in your case _. Since there's no such modifier hence the error. Change the code to:
$eregicheck = "/^([-!#\$%&'*+.\/0-9=?A-Z^_`a-z{|}~])+@([-!#\$%&'*+\/0-9=?A-Z^_`a-z{|}~]+\\.)+[a-zA-Z]{2,4}\$/";
P.S.: That seems like quite a complex regex, are you sure it can't be simplified? :P