1 error message at the time, custom regexp check on field

[eluser]CrustyDOD[/eluser]
Before i write my own 'addon' to the core, i'm just double checking that i haven't missed this somewhere in the guides and code..

1 error message at the time
Right now as far as i know (just started using CI) you have two options:
- Show ALL error messages at once
- Show error message per field (still shows more then just one error message)

I would like to show only one error message at the time by calling just one function. Is there a functions in the CI core right now that does this?

For example look at youtube sign up. If you miss one field, it shows just one error message for field you missed, and not ALL like CI is doing it right now.

custom regexp check on field
Is there a rule for custom regexp on a field?
Let's say i have drop down with values 1 and 2. I want to check that what someone submitted really is a value 1 or 2, and not 0, 3,4,5,... (faking form in this case).

Does this rule exist in the CI core?

Like i said, i'm just double checking cause i'm new to CI and i really like it.. and don't want to write something that already exist