Character class (square brackets) ignored while matching regex

Question

Why []([a].*) matches [](b)? I would expect it to require a to be the first character in ().

Answer 1

This is happening because Vim is actually seeing all the characters in between the very first [ and the last ] as a single character class that will match any one of ], (, [, or a.

In other words, what Vim is seeing here is []([a].*), with the bold part being the character class. So it's matching any of these four characters, followed by everything until a ).

This happens because, in order to match a literal ] in a character class, it needs to be the first character listed in the character class. So if you only have a [] an no other character class in the regexp (for example [](.*)), Vim will find that to be an invalid character class and match those characters literally. But as soon as you add another ] somewhere later in that regexp, you're now making it valid and making it match any character in between.

You can easily fix it by adding a backslash before the first [, to force it to match a literal bracket: \[]([a].*).