How would you write a regex that matches a number with commas for every three digits? It

Question:

How would you write a regex that matches a number with commas for every three digits? It must match the following:
• '42'
• '1,234'
• '6,368,745'
but not the following:
• '12,34,567' (which has only two digits between the commas)
• '1234' (which lacks commas)

Fantastic news! We've Found the answer you've been seeking!