How to have multiple regex for same element - regex

Question

I have a text box which has a regular expression which is something like below
^AB[a-zA-Z0-9]{20}$
which basically allows charecter AB , followed by 20 either alphabetic or numbers, and for example lets consider the validation error for not following this regex is Some Test Error
I have a scenario where user enters AB1234 and tabs out of the text box, and the error Some Test Error shows, but I have a requirement of not showing the same error message Some Test Error if user is trying to follow the format but not adhering to the entire regex.
Scenario 1 :- User enters CD12345675438976524381
I need to show Some Test Error
Scenario 2 : USer enters AB12345
I need to shoe Different Test Error, because user tried to enter a value starting from AB*
How can achieve this, is there a way of specifying multiple regex's?

Answer 1

I am not sure which language you are using... but I suppose that you may change the regex, when user got the message once. While the user is trying to enter the entire string, don't count the number, unless the user input the 21st char or something not belong to [a-zA-Z0-9]...
I wish I made myself understood, the point is that I suppose you change the regex in time.

Answer 2

I think you can for example use multiple regexes and check the input:
if input is valid, everithing is ok,
if input is invalid check: a) if starts with AB (regex: ^AB) or if is valid length (regex ^([^A][^B][a-zA-Z0-9]{20})$) show proper info
if is totally invalid, give another info
OR you can use one long regex, like:
^(AB[a-zA-Z0-9]{20})$|^(AB[a-zA-Z0-9]{0,19}|AB[a-zA-Z0-9]{21,})$|^([^A][^B][a-zA-Z0-9]{20})$
DEMO
which capture given type of input in saparete groups,
and then find which groups was captured to check level of correctness:
if group 1 exist - valid string,
if group 2 - starts with AB but inproper length,
if group 3 - proper lenght, invalid beginning
I sure there are also other solutions.