Tip: All the special variables are exactly one character long to avoid interfering with operators. (e.g. Is "$<>" one variable or "$<" followed by the numerical less-than operator?)

There's an exception to that rule. Variables whose name starts with "^" can have \w characters. To avoid confusion, the curlies around the name aren't optional for those variables. (e.g "$a" can also be written as "${a}", but the curlies can't be omitted for "${^PREMATCH}".)