Validate User Input Function | Dataford Interview Questions

The CoBank Customer Portal needs a lightweight validator for usernames before registration or profile updates. Implement a function that returns whether a proposed username satisfies all required formatting rules.

Formal Specification

Given a string username, return True if it is valid and False otherwise. A valid username must:

Contain between 3 and 20 characters, inclusive.
Start with an uppercase or lowercase English letter.
Contain only English letters, digits, and underscores.
Contain at least one character. The length requirement already excludes empty input.

Do not modify the input string. Treat uppercase and lowercase letters as valid without converting the username.

Constraints

0 <= len(username) <= 10^5
3 <= len(valid_username) <= 20
Input contains printable ASCII characters
Only letters, digits, and underscores are allowed
The first character must be an English letter

Function Signature

def is_valid_username(username):

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