An underscore, a dot and a dash must always be followed by one or more alphanumeric characters without using Regex

Question

I'm working on a school assignment that validates emails without using Regex. The premise of this exercise is to learn about methods and to practice our critical thinking. I understand that the code can be reduced to fewer lines.

Right now, I need to check all conditions for the prefix of an email (The characters before '@'):

• It contains at least one character.
• It contains only alphanumeric characters, underscores(‘_’), periods(‘.’), and dashes(‘-’).
• An underscore, a period, or a dash must always be followed by one or more alphanumeric characters.
• The first character must be alphanumeric.

Examples of valid prefixes are: “abc-d”, “abc.def”, “abc”, “abc_def”.
Examples of invalid prefixes are: “abc-”, “abc..d”, “.abc”, “abc#def”.

I'm having a hard time figuring out the third condition. So far, I have these methods that meet the other conditions.

public static boolean isAlphanumeric(char c) {
    return Character.isLetterOrDigit(c);
}

public static boolean isValidPrefixChar(char preChar) {
    char[] prefixChar = new char[] {'-', '.', '_'};
    
    for (int i = 0; i < prefixChar.length; i++) {
        if (prefixChar[i] == preChar) {
            return true;
        } else if (isAlphanumeric(preChar)) {
            return true;
        }
    }
    return false;
}

public static boolean isValidPrefix(String emailPrefix) {
    boolean result = false;
    
    // To check if first character is alphanumeric
    if (isAlphanumeric(emailPrefix.charAt(0)) && emailPrefix.length() > 1) {
        for (int i = 0; i < emailPrefix.length(); i++) {
            // If email prefix respects all conditions, change result to true
            if (isValidPrefixChar(emailPrefix.charAt(i))) {
                result = true;
            } else {
                result = false;
                break;
            }
        }
    }
    return result;
}

Answer 1

Let's look at your list:

• It contains at least one character.
• It contains only alphanumeric characters, underscores(‘_’), periods(‘.’), and dashes(‘-’).
• An underscore, a period, or a dash must always be followed by one or more alphanumeric characters.
• The first character must be alphanumeric.

As you say, 1, 2, and 4 are easy. Here's what I would do. My first line would check length and return false if incorrect. I would then iterate over the characters. Inside the loop;

• set boolean lastWasSpecial = false.
• Check that it's a legal character (condition 2)
• If index == 0, check that it's alphanumeric (condition 4)
• If it's one of the specials:
  • If lastWasSpecial is set, return false
  • Set lastWasSpecial = true;
• else set lastWasSpecial = false again

Should be about 10 lines of easily-readable code.