What is meaning of [^] in Javascript regexps?
javascript, regex
Solution
According to the JavaScript specification (ES3 and ES5), `[^]` matches any single code unit, the same as `[\s\S]`, `[\0-\uffff]`, `(.|\s)` (don't use that; unlike the others, it relies on backtracking), etc. The difference from `.` is that the dot doesn't match the four newline code points (`\r`, `\n`, `\u2028`, and `\u2029`).
I don't recommend using `[^]` or `[]`, because they don't work consistently cross-browser, and they prevent your regexes from working in other programming languages. IE <= 8 and older versions of Safari use the traditional (non-JavaScript) regex behavior for empty character classes. Older versions of Opera reverse the correct JavaScript behavior, so that `[]` matches any code unit and `[^]` never matches. The traditional regex behavior is that a leading, unescaped `]` within a character class is treated as a literal character and does not end the character class.
If you use the XRegExp library, `[]` and `[^]` work correctly and consistently cross-browser. XRegExp also adds the `s` (aka dotall or singleline) flag that makes a dot match any code unit (the same as `[^]` in a browser that correctly follows the JavaScript spec).
Problem
`[^a]` means any character other than `a`, we know, but what does `[^]` (with no following characters) mean? Just as `-` loses its meaning of character range in cases such as `[-]`, I assumed that `[^]` would match the caret. I spent way too long debugging this problem, only to find out that at least in Chrome 19 it appears to match anything--in other words, be equivalent to `.`. Is there a spec applicable here or what is the expected behavior? Yes, I'm aware that I can and probably should use `[\^]`. This question is more in the nature of morbid curiosity.