Fix character escaping in Perl lexer

[pyrmont]

In single-quoted strings, the Perl lexer currently tokenises \ as an error if it is followed by any character other than a '. This is a bug. Perl accepts the use of \ in single-quoted strings.

In the course of fixing this, errors in the handling of character escaping in double-quoted strings was also addressed. In that case, $ and @ are also valid characters to escape. The use of \ can still produce Error tokens in double-quoted strings if followed by another character.

This fixes #1163.

[pyrmont]

@miparnisari I am very much out of my depth with Perl but tried to experiment with how it handles the \ in double-quoted strings by running perl -de1 at the command line and playing around a bit with print. If I run print "foo\bar", I get the output "fooar" which doesn't really make sense to me at all but makes me think that \ should be lexed as an error unless it's followed by a valid character to escape. What do you think?

[jeremychan]

Do you mean you get "foar"? \b is a backspace http://www.java2s.com/Code/Perl/String/BackslashEscapesinPerl.htm

[pyrmont]

@jeremychan Sorry, right you are. Thanks for the correction! I'll have a closer look at that list and see if I need to make any more changes :)

[pyrmont]

It seems surprisingly difficult to determine the valid escape sequences in a Perl string. Using a combination of the lists from the Perl regular expression documentation and the Encode::Escape CPAN module, I've added more robust escaping support.