RFC822: email address validation with regular expressions

Question

as you know, this is how we validate an email address:

(?:(?:\\r\\n)?[ \\t])*(?:(?:(?:[^()<>@,;:\\\\\".\\[\\] \\000-\\031]+(?:(?:(?:\\r\\n)?[ \\t]
)+|\         


        

Answer 1

If you want to understand what is going on, you should look at a decent module such as Email::Address and note how the pattern is built from its constituent pieces:

my $CTL            = q{\x00-\x1F\x7F};
my $special        = q{()<>\\[\\]:;@\\\\,."};

my $text           = qr/[^\x0A\x0D]/;

my $quoted_pair    = qr/\\$text/;

my $ctext          = qr/(?>[^()\\]+)/;
my ($ccontent, $comment) = (q{})x2;
for (1 .. $COMMENT_NEST_LEVEL) {
  $ccontent = qr/$ctext|$quoted_pair|$comment/;
  $comment  = qr/\s*\((?:\s*$ccontent)*\s*\)\s*/;
}
my $cfws           = qr/$comment|\s+/;

my $atext          = qq/[^$CTL$special\\s]/;
my $atom           = qr/$cfws*$atext+$cfws*/;
my $dot_atom_text  = qr/$atext+(?:\.$atext+)*/;
my $dot_atom       = qr/$cfws*$dot_atom_text$cfws*/;

my $qtext          = qr/[^\\"]/;
my $qcontent       = qr/$qtext|$quoted_pair/;
my $quoted_string  = qr/$cfws*"$qcontent+"$cfws*/;

my $word           = qr/$atom|$quoted_string/;

etc etc etc.

my $simple_word    = qr/$atom|\.|\s*"$qcontent+"\s*/;
my $obs_phrase     = qr/$simple_word+/;

my $phrase         = qr/$obs_phrase|(?:$word+)/;

my $local_part     = qr/$dot_atom|$quoted_string/;
my $dtext          = qr/[^\[\]\\]/;
my $dcontent       = qr/$dtext|$quoted_pair/;
my $domain_literal = qr/$cfws*\[(?:\s*$dcontent)*\s*\]$cfws*/;
my $domain         = qr/$dot_atom|$domain_literal/;