Support `\ ` (backslash space) as equivalent to space, at least in /x mode

[zackw]

The documentation for insignificant-whitespace mode says this:

If you wish to match against whitespace in this mode, you can still use \s, \n, \t, etc. For escaping a single space character, you can use its hex character code \x20 or temporarily disable the x flag, e.g., (?-x: ).

Most other regex engines that support /x mode (and in particular Perl, which invented it) accept \ (backslash space) as equivalent to space and treat it as significant within /x mode. This can be significantly more readable than either of the alternatives supported by Rust, compare

Regex.new(r"(?x)When\ in\ the\ course\ of\ human\ events")

Regex.new(r"(?x)When\x20in\x20the\x20course\x20of\x20human\x20events")

And it would also ease porting code from other languages if Rust supported this.

[BurntSushi]

Could you please provide a program that reproduces the undesirable behavior? \ is already supported when x mode is enabled. See: https://play.rust-lang.org/?gist=32be08ffca301141e19037e296767725&version=stable&mode=debug&edition=2015

[BurntSushi]

See also: #501 (comment)

[zackw]

Could you please provide a program that reproduces the undesirable behavior? \ is already supported when x mode is enabled.

I was just going by the documentation. Please consider this a request to change the documentation so it says r"(?x)When\ in\ the\ course\ of\ human\ events" will indeed behave the same way /When\ in\ the\ course\ of\ human\ events/x would in Perl. (If you want to continue recommending the use of \x20 instead, that's fine by me.)