Question

I have not found a regexp to do this. I need to validate the “Message-ID:” value from an email. It is similar to a email address validation regexp but much simpler, without most of the edge cases the email address allows, from rfc2822

msg-id          =       [CFWS] "<" id-left "@" id-right ">" [CFWS] 
id-left         =       dot-atom-text / no-fold-quote / obs-id-left
id-right        =       dot-atom-text / no-fold-literal / obs-id-right
no-fold-quote   =       DQUOTE *(qtext / quoted-pair) DQUOTE
no-fold-literal =       "[" *(dtext / quoted-pair) "]"

Let’s say the outter <> are optional. dot-atom-text and missing definitions can be found in rfc2822

I am not proficient in regex and I prefer to use an already tested one, if exists.

Answer 1

As I could not find any I ended up implementing it myself. It is not a proper validation as per RFC2822 but a good enough aproximation for now:

static String VALIDMIDPATTERN = "[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*";
private static Pattern patvalidmid = Pattern.compile(VALIDMIDPATTERN);

public static boolean isMessageIdValid(String midt) {
    String mid = midt;
    if (StringUtils.countMatches(mid, "<") > 1)
        return false;
    if (StringUtils.countMatches(mid, ">") > 1)
        return false;
    if (StringUtils.containsAny(mid, "<>")) {
        mid = StringUtils.substringBetween(mid, "<", ">");
        if (StringUtils.isBlank(mid)) {
            return false;
        }
    }
    if (StringUtils.contains(mid, "..")) {
        return false;
    }
    //extract from <>
    mid = mid.trim();
    //now validate
    Matcher m = patvalidmid.matcher(mid);
    return m.matches();
}