All I want is a simple mask for valid JavaScript numbers. I expect a bit of complexity in handling exponents, Hex and the Octal formats all in one, but I wasn’t expecting something this cryptic. I like code that you can simply read, whereas this monstrosity requires a full page of explanation.

/^[+-]?(0[0-7]+|0x[\da-f]+|((0|[1-9]\d*|0\d*[89]\d*)(\.\d*)?|\.\d+)(e[+-]?\d+)?)$/i

Blah!

EDIT: Thanks to Andrea its much improved.
Here’s what I think the regex should look like:

^ [+-]? (
     // Octal
     0[0-7]+ |
     // Hex
     0x[\da-f]+ | 
     (
          // Basics: 0, 0., 0.ddd, ddd, ddd. ddd.ddd
          (0 | [1-9]\d*) (\.\d*)? |
          // Special: decimal parsing of Octal format
          0\d*[89]\d* |
          // Starting with decimal .nnn
          \.\d+
     )
     // Exponent. Not for Octal or Hex
     (e[+-]?\d+)?
) $

It would compile down to the same thing but be much more readable.