fix(isCreditCard): anchor Mastercard regex to reject partial strings

[abhu85]

Fixes #2717

What

The Mastercard pattern in src/lib/isCreditCard.js used a top-level alternation with no grouping, so the ^ and $ anchors each bound to only one branch:

/^5[1-5][0-9]{2}|(222[1-9]|22[3-9][0-9]|2[3-6][0-9]{2}|27[01][0-9]|2720)[0-9]{12}$/

Because | has lower precedence than the anchors, this parses as two patterns:

1. ^5[1-5][0-9]{2} — start-anchored only, matches any string beginning with 51–55 regardless of length.
2. (222[1-9]|…|2720)[0-9]{12}$ — end-anchored only.

So isCreditCard("5108") returned true: "5108" matches branch 1 and also passes the Luhn check (5×2=10→1, +0, +1, +8 = 10). Fifty 4-digit Luhn-valid strings in the 51–55 range are wrongly accepted.

Fix

Wrap the alternation in a non-capturing group so both anchors apply to every branch, restoring the intended full 16-digit format:

/^(?:5[1-5][0-9]{2}|222[1-9]|22[3-9][0-9]|2[3-6][0-9]{2}|27[01][0-9]|2720)[0-9]{12}$/

After the fix isCreditCard("5108") returns false, and all previously-valid 16-digit Mastercard numbers (5-series and 2-series) continue to validate.

I audited the other card patterns in the file: only mastercard had a correctness bug. (unionpay has a redundant nested ^^ but is functionally correct; left untouched to keep this PR scoped.)

References

• The reported issue isCreditCard: Mastercard regex anchoring bug accepts 4-digit and partial strings #2717
• Mastercard BIN ranges: 51–55 (legacy) and 2221–2720 (2017 expansion), 16 digits.

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (not applicable)
• Tests written — "5108" added to the no-provider and Mastercard invalid fixtures
• References provided in PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7fdc788) to head (933fe81).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2741   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          114       114           
  Lines         2587      2587           
  Branches       656       656           
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  Hits          2587      2587           

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