"+" in email-adresses not allowed.

// CheckEmail
// 
// mailbox     =  addr-spec                    ; simple address
//             /  phrase route-addr            ; name & addr-spec
// 
// route-addr  =  "<" [route] addr-spec ">"
// 
// route       =  1#("@" domain) ":"           ; path-relative
// 
// addr-spec   =  local-part "@" domain        ; global address
// 
// local-part  =  word *("." word)             ; uninterpreted
//                                             ; case-preserved
// 
// domain      =  sub-domain *("." sub-domain)
// 
// sub-domain  =  domain-ref / domain-literal
// 
// domain-ref  =  atom                         ; symbolic reference
// 
// atom        =  1*<any CHAR except specials, SPACE and CTLs>
// 
// specials    =  "(" / ")" / "<" / ">" / "@"  ; Must be in quoted-
//             /  "," / ";" / ":" / "\" / <">  ;  string, to use
//             /  "." / "[" / "]"              ;  within a word.
// 
//                                             ; (  Octal, Decimal.)
// CHAR        =  <any ASCII character>        ; (  0-177,  0.-127.)
// ALPHA       =  <any ASCII alphabetic character>
//                                             ; (101-132, 65.- 90.)
//                                             ; (141-172, 97.-122.)
// DIGIT       =  <any ASCII decimal digit>    ; ( 60- 71, 48.- 57.)
// CTL         =  <any ASCII control           ; (  0- 37,  0.- 31.)
//                 character and DEL>          ; (    177,     127.)
// CR          =  <ASCII CR, carriage return>  ; (     15,      13.)
// LF          =  <ASCII LF, linefeed>         ; (     12,      10.)
// SPACE       =  <ASCII SP, space>            ; (     40,      32.)
// HTAB        =  <ASCII HT, horizontal-tab>   ; (     11,       9.)
// <">         =  <ASCII quote mark>           ; (     42,      34.)
// CRLF        =  CR LF
// 
// LWSP-char   =  SPACE / HTAB                 ; semantics = SPACE
// 
// linear-white-space =  1*([CRLF] LWSP-char)  ; semantics = SPACE
//                                             ; CRLF => folding
// 
// delimiters  =  specials / linear-white-space / comment
// 
// text        =  <any CHAR, including bare    ; => atoms, specials,
//                 CR & bare LF, but NOT       ;  comments and
//                 including CRLF>             ;  quoted-strings are
//                                             ;  NOT recognized.
// 
// quoted-string = <"> *(qtext/quoted-pair) <">; Regular qtext or
//                                             ;   quoted chars.
// 
// qtext       =  <any CHAR excepting <">,     ; => may be folded
//                 "\" & CR, and including
//                 linear-white-space>
// 
// domain-literal =  "[" *(dtext / quoted-pair) "]"
// 
// 
// 
// 
// dtext       =  <any CHAR excluding "[",     ; => may be folded
//                 "]", "\" & CR, & including
//                 linear-white-space>
// 
// comment     =  "(" *(ctext / quoted-pair / comment) ")"
// 
// ctext       =  <any CHAR excluding "(",     ; => may be folded
//                 ")", "\" & CR, & including
//                 linear-white-space>
// 
// quoted-pair =  "\" CHAR                     ; may quote any char
// 
// phrase      =  1*word                       ; Sequence of words
// 
// word        =  atom / quoted-string
// 

// mailbox     =  addr-spec                    ; simple address
//             /  phrase route-addr            ; name & addr-spec
// route-addr  =  "<" [route] addr-spec ">"
// route       =  1#("@" domain) ":"           ; path-relative
// addr-spec   =  local-part "@" domain        ; global address

// validate_email("insight\@bedrijfsnet.nl");

// boolean validate_email ([string email address])

// This function validates the format of an email address in a rather 
// exhaustive manner, based on the relevant RFC. Note: this does
// NOT validate the email address in any functional way. Just because
// it looks OK doesn't mean it works.

function validate_email ($eaddr="")
{

    if (empty($eaddr))
    {
// print "[$eaddr] is not valid\n";
        return false;
    }
    $laddr = "";
    $laddr = $eaddr;

//  if the addr-spec is in a route-addr, strip away the phrase and <>s

    $laddr = preg_replace('/^.*</','', $laddr);
    $laddr = preg_replace('/>.*$/','',$laddr);
    if (preg_match('/^\@.*:/',$laddr))    #path-relative domain
    {
        list($domain,$addr_spec) = preg_split('/:/',$laddr);
        $domain = preg_replace('/^\@/','',$domain);
        if (!is_domain($domain)) { return false; }
        $laddr = $addr_spec;
    }
    return(is_addr_spec($laddr));
}

function is_addr_spec ( $eaddr = "" )
{
    list($local_part,$domain) = preg_split('/\@/',$eaddr);
    if (!is_local_part($local_part) || !is_domain($domain))
    {
// print "[$eaddr] is not valid\n";
        return false;
    }
    else
    {
// print "[$eaddr] is valid\n";
        return true;
    }
}

// local-part  =  word *("." word)             ; uninterpreted
function is_local_part ( $local_part = "" )
{
    if (empty($local_part)) { return false; }

    $bit_array = preg_split('/\./',$local_part);
    while (list(,$bit) = each($bit_array))
    {
        if (!is_word($bit)) { return false; }
    }
    return true;
}

// word        =  atom / quoted-string
// quoted-string = <"> *(qtext/quoted-pair) <">; Regular qtext or
//                                             ;   quoted chars.
// qtext       =  <any CHAR excepting <">,     ; => may be folded
//                 "\" & CR, and including
//                 linear-white-space>
// quoted-pair =  "\" CHAR                     ; may quote any char
function is_word ( $word = "")
{

    if (preg_match('/^".*"$/i',$word))
    {
        return(is_quoted_string($word));
    }
    return(is_atom($word));
}

function is_quoted_string ( $word = "")
{
    $word = preg_replace('/^"/','',$word);    # remove leading quote
    $word = preg_replace('/"$/','',$word);    # remove trailing quote
    $word = preg_replace('/\\+/','',$word);    # remove any quoted-pairs
    if (preg_match('/\"\\\r/',$word))    # if ", \ or CR, it's bad qtext
    {
        return false;
    }
    return true;
}


// atom        =  1*<any CHAR except specials, SPACE and CTLs>
// specials    =  "(" / ")" / "<" / ">" / "@"  ; Must be in quoted-
//             /  "," / ";" / ":" / "\" / <">  ;  string, to use
//             /  "." / "[" / "]"              ;  within a word.
// SPACE       =  <ASCII SP, space>            ; (     40,      32.)
// CTL         =  <any ASCII control           ; (  0- 37,  0.- 31.)
//                 character and DEL>          ; (    177,     127.)
function is_atom ( $atom = "")
{

    if (
    (preg_match('/[\(\)\<\>\@\,\;\:\\\"\.\[\]]/',$atom))    # specials
        || (preg_match('/\040/',$atom))            # SPACE
        || (preg_match('/[\x00-\x1F]/',$atom))        # CTLs
    )
    {
        return false;
    }
    return true;
}

// domain      =  sub-domain *("." sub-domain)
// sub-domain  =  domain-ref / domain-literal
// domain-ref  =  atom                         ; symbolic reference
function is_domain ( $domain = "")
{

    if (empty($domain)) { return false; }

//  this is not strictly required, but is 99% likely sign of a bad domain
    if (!preg_match('/\./',$domain)) { return false; }

    $dbit_array = preg_split('/./',$domain);
    while (list(,$dbit) = each($dbit_array))
    {
        if (!is_sub_domain($dbit)) { return false; }
    }
    return true;
}
function is_sub_domain ( $subd = "")
{
    if (preg_match('/^\[.*\]$/',$subd))    #domain-literal
    {
        return(is_domain_literal($subd));
    }
    return(is_atom($subd));
}
// domain-literal =  "[" *(dtext / quoted-pair) "]"
// dtext       =  <any CHAR excluding "[",     ; => may be folded
//                 "]", "\" & CR, & including
//                 linear-white-space>
// quoted-pair =  "\" CHAR                     ; may quote any char
function is_domain_literal ( $dom = "")
{
    $dom = preg_replace('/\\+/','',$dom);        # remove quoted pairs
    if (preg_match('/[\[\]\\\r]/',$dom))    # bad dtext characters
    {
        return false;
    }
    return true;
}

// void print_validate_email ([string email address])

// This function prints out the result of calling the validate_email()
// function on a given email address.

function print_validate_email ($eaddr="")
{
    $result = validate_email($eaddr) ? "is valid" : "is not valid";
    print "<h4>email address (".htmlspecialchars($eaddr).") $result</h4>\n";
}