RegEx match another pattern if first pattern fails

Question

Here's the logic I'm failing with: If the input is 14 characters, return characters 9 through 13. Otherwise, return everything.

(?<=^.{8}).{5}(?=.$) works as I'd expect, passing with characters 9 through 13 as the result set. If I append |.* to the RegEx to make (?<=^.{8}).{5}(?=.$)|.* , it always returns everything. I'm obviously going about this incorrectly.

Any input?

Answer 1

^(?=.{14}$).{8}(.*).$|^.*$

Try this .This will return characters from 9 to 13 when string is of 14 characters.See demo.

https://regex101.com/r/pT4tM5/29

Answer 2

我确定正则表达式是必需的，但是由于您仅处理字符串长度，因此也可以使用子字符串

input.Length == 14 ? return input.Substring(8,5) : input;