What does a character class with only a lone caret do?

[^][…] is not two character classes but just one character class containing any other character except ], [, and … (see Special Characters Inside a Bracketed Character Class):

However, if the ] is the first (or the second if the first character is a caret) character of a bracketed character class, it does not denote the end of the class (as you cannot have an empty class) and is considered part of the set of characters that can be matched without escaping.

Examples:

"+"   =~ /[+?*]/     #  Match, "+" in a character class is not special.
"\cH" =~ /[\b]/      #  Match, \b inside in a character class
                     #  is equivalent to a backspace.
"]"   =~ /[][]/      #  Match, as the character class contains.
                     #  both [ and ].
"[]"  =~ /[[]]/      #  Match, the pattern contains a character class
                     #  containing just ], and the character class is
                     #  followed by a ].
 

Character Classes or Character Sets

With a “character class”, also called “character set”, you can tell the regex engine to match only one out of several characters. Simply place the characters you want to match between square brackets. If you want to match an a or an e, use [ae]. You could use this in gr[ae]y

A character class matches only a single character. gr[ae]y does not match graay, graey or any such thing. The order of the characters inside a character class does not matter. The results are identical.

You can use a hyphen inside a character class to specify a range of characters. [0-9] matches a single digit between 0 and 9. You can use more than one range. [0-9a-fA-F] matches a single hexadecimal digit, case insensitively. You can combine ranges and single characters. [0-9a-fxA-FX] matches a hexadecimal digit or the letter X. Again, the order of the characters and the ranges does not matter.

Character classes are one of the most commonly used features of regular expressions. You can find a word, even if it is misspelled, such as sep[ae]r[ae]te or li[cs]en[cs]e. You can find an identifier in a programming language with [A-Za-z_][A-Za-z_0-9]*. You can find a C-style hexadecimal number with 0[xX][A-Fa-f0-9]+.

Negated Character Classes

Typing a caret after the opening square bracket negates the character class. The result is that the character class matches any character that is not in the character class. Unlike the dot, negated character classes also match (invisible) line break characters. If you don’t want a negated character class to match line breaks, you need to include the line break characters in the class. [^0-9\r\n] matches any character that is not a digit or a line break.

It is important to remember that a negated character class still must match a character. q[^u] does not mean: “a q not followed by a u”. It means: “a q followed by a character that is not a u”. It does not match the q in the string Iraq. It does match the q and the space after the q in Iraq is a country. Indeed: the space becomes part of the overall match, because it is the “character that is not a u” that is matched by the negated character class in the above regexp. If you want the regex to match the q, and only the q, in both strings, you need to use negative lookahead: q(?!u). But we will get to that later.