Validate complex string input (fixed length pattern)

Question

What is the best way to validate this type of input:

1234,4589,7889

Only allowed numbers. Each number must contain 4 digits. They are separated by ,. There can be one or many. Last item does not have a separator.

Answer 1

• Each number must contain 4 digits : \d{4}
• They are separated by "," : \d{4},\d{4}
• There can be one or many : (?:\d{4},)+
• Last item does not have a separator => aka 4 digits followed by any number of comma + 4 digits : \d{4}(?:,\d{4})*

^\d{4}(?:,\d{4})*$ to make sure the pattern cover the start and the end of the string.