Before you start: test every pattern with examples that should match and examples that should fail.
DD/MM/YYYY structure
A purely structural pattern requires two digits, slash, two digits, slash, and four digits.
Regex: ^\d{2}\/\d{2}\/\d{4}$Why 99/99/9999 still matches
\d{2} only knows there are two digits. It does not know the valid month range or leap-year rules.
Validate calendar meaning in code
After checking format, parse the value with a date library or native date type and confirm that it represents a valid date.
Consider ISO dates for APIs
YYYY-MM-DD is common in APIs and reduces regional ambiguity. The correct Regex should follow the contract of your system.
Next step: test it for real
Use the Playground to experiment with the pattern, then practice in the game with feedback on false positives and false negatives.