Pattern matching regular expressions

Question

I'm attempting to determine if a string matches a pattern of containing less than 100 alphanumeric characters a-z or 0-9 case insensitive. So my regular expression string looks like:
"^[a-zA-Z0-9]{0,100}$"And I use something like...
Pattern pattern = Pattern.compile( regexString );I'd like to modify my regex string to include the email 'at' symbol "@". So that the at symbol will be allowed. But my understanding of regex is very limited. How do I include an "or at symbol" in my regex expression?
Thanks for your help.

Answer 1

* Code by sabre150
private static final Pattern emailMatcher;
    static
        // Build up the regular expression according to RFC821
        // http://www.ietf.org/rfc/rfc0821.txt
        // <x> ::= any one of the 128 ASCII characters (no exceptions)
        String x_ = "\u0000-\u007f";
        // <special> ::= "<" | ">" | "(" | ")" | "[" | "]" | "\" | "."
        //              | "," | ";" | ":" | "@"  """ | the control
        //              characters (ASCII codes 0 through 31 inclusive and
        //              127)
        String special_ = "<>()\\[\\]\\\\\\.,;:@\"\u0000-\u001f\u007f";
        // <c> ::= any one of the 128 ASCII characters, but not any
        //             <special> or <SP>
        String c_ = "[" + x_ + "&&" + "[^" + special_ + "]&&[^ ]]";
        // <char> ::= <c> | "\" <x>
        String char_ = "(?:" + c_ + "|\\\\[" + x_ + "])";
        // <string> ::= <char> | <char> <string>
        String string_ = char_ + "+";
        // <dot-string> ::= <string> | <string> "." <dot-string>
        String dot_string_ = string_ + "(?:\\." + string_ + ")*";
        // <q> ::= any one of the 128 ASCII characters except <CR>,
        //               <LF>, quote ("), or backslash (\)
        String q_ = "["+x_+"$$[^\r\n\"\\\\]]";
        // <qtext> ::=  "\" <x> | "\" <x> <qtext> | <q> | <q> <qtext>
        String qtext_ = "(?:\\\\[" + x_ + "]|" + q_ + ")+";
        // <quoted-string> ::=  """ <qtext> """
        String quoted_string_ = "\"" + qtext_ + "\"";
        // <local-part> ::= <dot-string> | <quoted-string>
        String local_part_ = "(?:(?:" + dot_string_ + ")|(?:" + quoted_string_ + "))";
        // <a> ::= any one of the 52 alphabetic characters A through Z
        //              in upper case and a through z in lower case
        String a_ = "[a-zA-Z]";
        // <d> ::= any one of the ten digits 0 through 9
        String d_ = "[0-9]";
        // <let-dig> ::= <a> | <d>
        String let_dig_ = "[" + a_ + d_ + "]";
        // <let-dig-hyp> ::= <a> | <d> | "-"
        String let_dig_hyp_ = "[-" + a_ + d_ + "]";
        // <ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>
        // String ldh_str_ = let_dig_hyp_ + "+";
        // RFC821 looks wrong since the production "<name> ::= <a> <ldh-str> <let-dig>"
        // forces a name to have at least 3 characters and country codes such as
        // uk,ca etc would be illegal! I shall change this to make the
        // second term of <name> optional by make a zero length ldh-str allowable.
        String ldh_str_ = let_dig_hyp_ + "*";
        // <name> ::= <a> <ldh-str> <let-dig>
        String name_ = "(?:" + a_ + ldh_str_ + let_dig_ + ")";
        // <number> ::= <d> | <d> <number>
        String number_ = d_ + "+";
        // <snum> ::= one, two, or three digits representing a decimal
        //              integer value in the range 0 through 255
        String snum_ = "(?:[01]?[0-9]{2}|2[0-4][0-9]|25[0-5])";
        // <dotnum> ::= <snum> "." <snum> "." <snum> "." <snum>
        String dotnum_ = snum_ + "(?:\\." + snum_ + "){3}"; // + Dotted quad
        // <element> ::= <name> | "#" <number> | "[" <dotnum> "]"
        String element_ = "(?:" + name_ + "|#" + number_ + "|\\[" + dotnum_ + "\\])";
        // <domain> ::=  <element> | <element> "." <domain>
        String domain_ = element_ + "(?:\\." + element_ + ")*";
        // <mailbox> ::= <local-part> "@" <domain>
        String mailbox_ = local_part_ + "@" + domain_;
        emailMatcher = Pattern.compile(mailbox_);
        System.out.println("Email address regex = " + emailMatcher);
    }Wow. Sheesh, sabre150 that's pretty impressive. I like it for two reasons. First it avoids some false negatives that I would have gotten using the regex I mentioned. Like, [email protected] is a valid email address which my regex pattern has rejected and yours accepts. It's unusual but it's valid. And second I like the way you have compartmentalized each rule so that changes, if any custom changes are desired, are easier to make. Like if I want to specifically aim for a particular domain for whatever reason. And you've commented it so that it is easier to read, for someone like myself who knows almost nothing about regex.
Thanks, Good stuff!