Question

Can I use

\d\d\d\d[^\d]

to check for four consecutive digits?

For example,

OK:

• 411112
• 455553
• 1200003
• f44443
• g55553
• 3333

No:

• f4442
• 45553
• f4444g4444
• f44444444

Answer 1

If you want to find any series of 4 digits in a string /\d\d\d\d/ or /\d{4}/ will do. If you want to find a series of exactly 4 digits, use /[^\d]\d{4}[^\d]/. If the string should simply contain 4 consecutive digits use /^\d{4}$/.

Edit: I think you want to find 4 of the same digits, you need a backreference for that. /(\d)\1{3}/ is probably what you’re looking for.

Edit 2: /(^|(.)(?!\2))(\d)\3{3}(?!\3)/ will only match strings with exactly 4 of the same consecutive digits.

The first group matches the start of the string or any character. Then there’s a negative look-ahead that uses the first group to ensure that the following characters don’t match the first character, if any. The third group matches any digit, which is then repeated 3 times with a backreference to group 3. Finally there’s a look-ahead that ensures that the following character doesn’t match the series of consecutive digits.

This sort of stuff is difficult to do in javascript because you don’t have things like forward references and look-behind.