No ambiguity flagged — the rule fully specifies the accepted formats and digit count. Pattern: /^(\(\d{3}\)\s?|\d{3}[-.]?)\d{3}[-.]?\d{4}$/ Walkthrough: - (555) 123-4567 — PASS. Matches the (\d{3}\)\s? branch, then 123 + - + 4567. - 555-123-4567 — PASS. Matches the \d{3}[-.]? branch with - separators throughout. - 555.123.4567 — PASS. Same branch with . separators. - 5551234567 — PASS. Separators are optional (?), so bare digits satisfy it. - 555-12-4567 — FAIL, correctly. Only 2 digits in the middle group instead of 3, so it doesn't match \d{3} after the first separator. - 12345 — FAIL, correctly. Only 5 digits total, far short of the required 10. Segment breakdown: - ^ — anchors to the start of the string, so no leading junk is allowed. - (\(\d{3}\)\s?|\d{3}[-.]?) — the area code, either parenthesized with an optional trailing space, or three bare digits with an optional dash/dot. - \d{3}[-.]? — the exchange (middle 3 digits) with an optional separator. - \d{4} — the last 4 digits, no separator expected after these. - $ — anchors to the end, so no trailing junk is allowed. Edge cases not in the samples: 1. "555 123 4567" (space-separated) — this pattern does NOT match it, since only - and . are treated as valid separators, not a bare space. Flag this if space-formatted numbers are expected in real data. 2. "+1-555-123-4567" (with country code) — does NOT match, since the pattern anchors to the start and doesn't account for a +1 prefix. Would need a separate optional country-code group if that's in scope. 3. "555-123-4567 ext 202" — does NOT match as a full-string pattern because of the trailing $ anchor; would need the anchor relaxed (or a \b boundary approach) if extensions must be tolerated.