Translate a Regex Pattern Into a Plain English Explanation

Someone left a regular expression in the codebase with no comment, and reading it symbol by symbol is not how you want to spend the next twenty minutes. This breaks it into its parts in order, explains what each one matches in plain English, and gives one example that matches and one that does not, so you can trust it before you touch it.

Explain what this regular expression actually matches, in plain English, as if to a colleague who can read code but has never learned regex syntax properly.

{REGEX_PATTERN}

Break the explanation into the distinct parts of the pattern, in the order they appear, and give one example string that would match and one that would not.