User name regex not working when checking for length

Web User :

I am trying to validate user names, in Java, based on the following rules:

• must start with lowercase alpha (a-z)
• subsequent characters can be lowercase alpha (a-z) or digits (0-9)
• Only one symbol, period "." may be used, only once and cannot be at the start or end of the user name

The regex I am using to perform validation is ^[a-z]+\.?[a-z0-9]+$

This works quite nice (there may be better ways of doing this), except now I want to allow user names that are between 3 and 10 characters long. Anywhere I try to use {3,10}, e.g. ^([a-z]+\.?[a-z0-9]+){3,10}$, validation fails. I am using an excellent visual regex tool and online regex tester.

The code itself is really simple; I am using the String class' matches method in Java 8. john.doe passes the regex and length validation, but j.doe does not.

Update, based on selected answer:

The Java code can be a little self-explanatory, given the complexity of the regex:

private static final String PATTERN_USERNAME_REGEX = new StringBuilder()
    // the string should contain 3 to 10 chars
    .append("(?=.{3,10}$)")
    // the string should start with a lowercase ASCII letter
    .append("[a-z]")
    // then followed by zero or more lowercase ASCII letters or/and digits
    .append("[a-z0-9]*")
    // an optional sequence of a period (".") followed with 1 or more lowercase ASCII letters
    // or/and digits (that + means you can't have . at the end of the string and ? guarantees
    // the period can only appear once in the string)
    .append("(?:\\\\.[a-z0-9]+)?")
    .toString();

Wiktor Stribiżew :

The regex you seek is

^(?=.{3,10}$)[a-z][a-z0-9]*(?:\.[a-z0-9]+)?$

In Java,

s.matches("(?=.{3,10}$)[a-z][a-z0-9]*(?:\\.[a-z0-9]+)?")

See the regex demo.

Details

• ^ - start of string (no need to use in String#matches)
• (?=.{3,10}$) - the string should contain 3 to 10 chars
• [a-z] - a lowercase ASCII letter
• [a-z0-9]* zero or more lowercase ASCII letters or/and digits
• (?:\.[a-z0-9]+)? - an optional sequence of a . followed with 1 or more lowercase ASCII letters or/and digits (that + means you can't have . at the end of the string and ? guarantees the . can only appear once in the string)
• $ - end of string (no need to use in String#matches)