Regex builder

Builds a regular expression from examples and explains every part.

Write a regular expression for {{LANGUAGE/ENGINE: e.g. JavaScript, Python, PCRE}} that matches: {{WHAT}}.

Should match:
{{EXAMPLES THAT MUST MATCH}}

Should NOT match:
{{EXAMPLES THAT MUST NOT MATCH}}

Give me:
1. The regex.
2. A breakdown of each part.
3. A test table showing each example and whether it matches.
4. Edge cases it will get wrong, if any, and a simpler non-regex alternative if regex is the wrong tool.