Side note about debuggex.com itself: The "Phone Number" example is a bad idea. Please either remove it or otherwise relabel it to warn against using it.
Rationale: Almost everywhere in the world outside of US and Canada, phone numbers do not look like that, and interacting with websites that assume they do is frustrating. Even USians or Canadians may want to enter a phone number with an extension, which would get rejected by that regular expression (a colleague of mine got frustrated to no end by websites and bureaucracy assuming the 3-3-4 style of phone numbers when she moved to Canada, because she prefers not to use a cell phone too much, and her work number has an extension).
Rationale: Almost everywhere in the world outside of US and Canada, phone numbers do not look like that, and interacting with websites that assume they do is frustrating. Even USians or Canadians may want to enter a phone number with an extension, which would get rejected by that regular expression (a colleague of mine got frustrated to no end by websites and bureaucracy assuming the 3-3-4 style of phone numbers when she moved to Canada, because she prefers not to use a cell phone too much, and her work number has an extension).