Question

I need help forming a regular expression to check if the input string is ONLY of the pattern 1 or 2 alphabets (can be lower or uppercase) followed by 1 or 2 digits. Valid strings would be d1,d15,ha1,ha20 so on.

Answer 1

The following should do what you want:

\A[a-zA-Z]{1,2}\d{1,2}\z

[a-zA-Z] is a character class that matches any letter, \d is equivalent to [0-9] and matches any digit, and {1,2} means “repeat the previous element 1 or 2 times”.

\A and \z are anchors, and they only match at the beginning and end of a string respectively (they don’t match any characters, they just require the string to start or end at them to allow a match).

You will also commonly see the anchors ^ and $, I used \A and \z because $ will match just before a newline at the end of the string and can have its behavior modified by options, whereas \z always means the very end of the string.

The following page gives a nice summary on regular expression syntax:
http://www.regular-expressions.info/reference.html