Question

I am writing a regular expression in which the string can be of 2-3 characters.
The first character has to be a Alphabet between A and H (capitals). This character has to be followed by a number between 1 and 12.
I wrote

[A-H]{1}[1-12]{1,2}

This is fine when I keyin A12 but not when I keyin A6

Please suggest.

Answer 1

You can’t specify a range of digits like that because it is implemented as a range between characters, so [1-12] is equivalent to [12], which would only match either a 1 or a 2. Instead, try the following:

[A-H](?:1[012]|[1-9])

Here is an explanation:

[A-H]        # one letter from A to H
(?:          # start non-capturing group
   1[012]      # 1 followed by 0, 1, or 2 (10, 11, 12)
   |           # OR
   [1-9]       # one digit from 1 to 9
)            # end non-capturing group

Note that the {1} after [A-H] in your original regex is unnecessary, [A-H]{1} and [A-H] are equivalent.

You may want to consider adding anchors to the regex, otherwise you would also get a partial match on a string like A20. If you are trying to match an entire string then you should use the following:

\A[A-H](?:1[012]|[1-9])\z

If it is within a larger text you could use word boundaries instead:

\b[A-H](?:1[012]|[1-9])\b