[ruby-talk:5078] Regex.c error reporting beautifier

A patch (as an attachment) for friendlier regexp compilation problem
reporting. To be precice, now Ruby tells better what's wrong with following
cases.

There's still some work to do. Two cases of invalid_pattern, that I didn't
understand how to generate. And I didn't check the other error messages - I
expect they're quite good already.

The patch shouldn't be applied directly. The reason is that for some
formatting with snprintf I needed a buffer, and I think it does not get
cleaned up as I didn't knew how to clean it up.

Anyway, all comments are welcome.

   - Aleksi


./ruby -e'p /++/.match("+++")[0]'
Previously:
  -e:1: invalid regular expression: /++/
Now:
  -e:1: invalid regular expression; there's no previous pattern, 
  to which '+' would define cardinality at 1: /++/


ruby -e'p /[/.match("foobar [")[0]'
Previously:
  -e:1: invalid regular expression: /[/
Now:
  -e:1: invalid regular expression; '[' can't be the last character: /[/


ruby -e'p /[]/.match("")[0]'
Previously:
  -e:1: invalid regular expression: /[]/
Now:
  -e:1: invalid regular expression; empty range: /[]/


/ruby -e'p /fo[[:/.match("fo[[:")[0]'
Previously:
  -e:1: invalid regular expression: /fo[[:/
Now:
  -e:1: invalid regular expression; re can't end '[[:': /fo[[:/


ruby -e'p /[[:alpha:]-a]/.match("")[0]'
Previously:
  -e:1: invalid regular expression: /[[:alpha:]-a]/
Now:
  -e:1: invalid regular expression; can't use character class as a start
value of range: /[[:alpha:]-a]/


ruby -e'p /[[:foo:]]/.match("foo")[0]'
Previously:
  -e:1: invalid regular expression: /[[:foo:]]/
Now:
  -e:1: invalid regular expression; foo is not a character class:
/[[:foo:]]/


ruby -e'p /[[:alnum:]/.match("")[0]'
Previously:
  -e:1: invalid regular expression: /[[:alnum:]/
Now:
-e:1: invalid regular expression; range doesn't have ending ']' after a
character class: /[[:alnum:]/


ruby -e'p /foo{/.match("foo")[0]'
Previously:
  -e:1: invalid regular expression: /foo{/
Now:
  -e:1: invalid regular expression; '{' can't be last character: /foo{/


ruby -e'p /{a/.match("oo")[0]'
Previously:
  -e:1: invalid regular expression: /{a/
Now:
  -e:1: invalid regular expression; there's no previous pattern, to which
'{' would define cardinality at 1: /{a/


ruby -e'str="\\"; p %r{#{str}}.match("")[0]'
Previously:
  -e:1: invalid regular expression: /\/ (RegexpError)
Now:
  -e:1: invalid regular expression; '\' can't be last character: /\/
(RegexpError)