Regular Expression in Java for validating username

Question

I\'m trying the username chains in Java with following rules:

• Length >=3
• Valid characters: a-z, A-Z, 0-9, points, dashes and underscores.

Answer 1

BTW, if there is an extra requirement: the starting letter of the username must be a character, you can write

try {
    if (subjectString.matches("\\b[a-zA-Z][a-zA-Z0-9\\-._]{3,}\\b")) {
        // Successful match 

    } else {
        // No match 

    }
} catch (PatternSyntaxException ex) {
    // Invalid regex 

}

Based on an example here.