regex - What will be the regular expression for a fixed format of alphanumeric value -

I have a problem where I need to see that the text box entry is an alphanumeric value this entry character (AZ) ) Is the number (0-9) format, where only one alphabet and numbers can be as much as they want. The lipabat should always come first. Must enter a minimum of 2 characters. Example A100 is A1, it should not accept 1A.

  ^ [az] [0-9] + $  

the above mentioned instrument, the beginning of the string Begins with, set one letter in A_Z, after one or more characters in set 0-9, after the end of the string.