Regex to match chars at the start and digits at the end

Question

I need a regular expression for the following conditions:

• The length of the string is exactly 8
• The string may not start with PAK
• The first character must be an upper case char [AZ]
• The second to fourth char can be an upper case char
• The remaining chars may only be digits

Examples:

• PAK12345 -> not allowed
• T1234567 -> allowed
• ABCD1234 -> allowed
• ABC12345 -> allowed
• AB12345A -> not allowed
• ABCDE123 -> not allowed
• ABC123 -> not allowed

Actually I have the following regex:

(?!PAK)([A-Z]{1,4}[A-Z0-9]{7})$

The problem with this regex is, that "ABCDE123" is a match.

How can I say, that the first 1 to 4 characters are only upper case chars and the remaining (until the total lenght of 8) are digits?

Answer 1

Remove the regex range operator and do an exact character length match. Add a positive lookahead at the very first to ensure that the length must be exactly 8.

^(?=.{8}$)(?!PAK)([A-Z]{1,4}[0-9]+)$

DEMO