I'll do one better and explain the whole thing.
for pattern:
/ has been selected as the pattern delimiter, this particular mark starts the pattern (with the following character.) This character can be any symbol. /, @, and # are often used mostly because they don't appear as characters to match in the patterns too often.
A simply match a capital a
[ this is a metacharacter. It's used as a character class starting mark. The contents following it are allowed in any order matching a single instance, unless other modified.
A-Z match any capital letter
] stops the character class.
* a match modifer. This particular metacharacter matches the previous object (character, character class, grouping) zero or more times unless modified by a
?
? a match modifer. When not after a
* or
+ modifer, it will work against the previous object (character, character class, grouping) to find zero or one instance. When after a
* or
+ it will tell the metacharacter to match the shortest possible set that satisfies the pattern. (Behaviour is reversed if the ungreedy pattern modifier is in effect.)
B match a capital b.
/ is now the ending of the pattern space. The next character(s) are entire pattern modifiers.
Putting it all in plain english: find a capital 'a' followed by any number of other capital letters to the closest capital 'b' anywhere in the string.