Match capital letters and fill in with subsequent letters until certain string length

Question

I have a camel-cased string, for example: JustAString.

I would like to form strings of length 4 by following these rules:

• grab all capital letters;
• if more than 4 capital letters, keep only the first 4;
• if less than 4 capital letters, capitalize and add letters that follow the last capital letter, until the length becomes 4.

Here are the 3 cases that may happen:

• ThisIsMyString will yield TIMS (capitals);
• ThisIsOneVeryLongString will yield TIOV (first 4 capitals);
• MyString will yield MSTR (capitals + tr capitalized).

I managed to solve the first two cases with this snippet:

str.scan(/[A-Z]/).first(4).join

However, I am not quite sure how can I best modify the above snippet to handle the last case also (or even try something different).

P.S.: The string is guaranteed to have at least a capital and 4 characters. However, if theoretically a capital is missing, the first 4 characters should be taken into consideration. If there aren't 4 characters, the missing characters may be filled in with the first alphabet characters (abcd). But, as mentioned, these 2 edge cases won't normally happen.

Answer 1

Replace any character(s) that precedes a capital with nothing, then grab the first 4 characters and upcase:

str.gsub(/[^A-Z]+([A-Z])/){$1}[0..3].upcase

This handles no capitals too. As for the edge case of not enough chars, you could append "abcd" but I'd find it cleaner just to do this in a separate line after the fact: output_string = (output_string + "abcd")[0..3] if output_string.length < 4. This is reads much cleaner and performs (inconsequentially) better if this is really a rare edge case.