Username rules with text input

A sign-up page checks usernames before creating an account. Write tests that expose every bug in ten buggy implementations.

The checker is a single regular expression, which is how most real validators are written, and each buggy copy changes it in one realistic way: a quantifier that is off by one, a character class that is too wide or too narrow, a missing anchor. Unlike a number, a username has two things to test at once. Its length behaves like a range, so it has boundaries. Its characters behave like sets, so each kind of character needs its own example. A test such as alice is valid in every way and finds nothing, so think about what a developer could have changed in the pattern and choose names that would behave differently.

Requirements

• The username has 3 to 15 characters.
• Allowed characters are letters (upper or lower case), digits and underscores.
• The first character must be a letter or an underscore, not a digit.
• The result is valid or invalid. Do not use a | character in a test value.