Question

I need regex that validates a string that:

• has 4 or 5 letters
• then one underscore
• then either an I or O (only one of them)
• then one underscore
• then 4 numeric digits

Now here where it complicates further.
After all that it may have:

• another underscore
• then 4 numeric digits.

In the end all these must be valid:

ASDF_I_0002

ASDFG_O_0003_0324

ASDF_O_0001

ASDF_I_0001_0001

Thank you.

Answer 1

• has 4 or 5 letters (A-Z)

  [A-Z]{4,5}
  

• then one underscore

  [A-Z]{4,5}_
  

• then either an I or O (only one of them)

  [A-Z]{4,5}_[IO]
  

• then one underscore

  [A-Z]{4,5}_[IO]_
  

• then 4 numeric digits

  [A-Z]{4,5}_[IO]_[0-9]{4}
  

After all that it may have:

    [A-Z]{4,5}_[IO]_[0-9]{4}()?

• another underscore

  [A-Z]{4,5}_[IO]_[0-9]{4}(_)?
  

• then 4 numeric digits.

  [A-Z]{4,5}_[IO]_[0-9]{4}(_[0-9]{4})?
  

You’ve lined out your requirements so nicely, I wonder where the problem was to make a regex from them. 😉