javascript - Crockford - Chapter 7 - parse_url

Question

var parse_url = /^(?:([A-Za-z]+):)?(\/{0,3})([0-9.\-A-Za-z]+)(?::(\d+))?(?:\/([^?#]*))?(?:\?([^#]*))?(?:#(.*))?$/;

Why is the dot . in this part [0-9.-A-Za-z]+ not escaped by a backslash?

Answer 1

Brackets ([]) specify a character class: matching a single character in the string between [].

While inside a character class, only the \ and - have special meaning (are metacharacters):

• backslash \: general escape character.
• hyphen -: character range.
  • Notice, though, it must be between chars to have special meaning:
    • [0-9] means any number between 0 and 9, while in [09-], - assumes the quality of an ordinary -, not a range.

That's why, inside [], a . is just (will only match) a dot.

Note: It is also worth noticing that the char ] must be escaped to be used inside a character class, such as [a-z\]], otherwise it will close it as usual. Finally, using ^, as in [^a-z], designates a negated character class, that means any char that is not one of those (in the example, any char that is not a...z).

Answer 2

So it matches a dot.

Except under some circumstances (e.g., escaping the range hyphen when it's not the first character in the character class brackets) you don't need to escape special characters in a class.

You may escape the normal metacharacters inside character classes, but it's noisy and redundant.