Fix parse error of system default /usr/share/nano/json.nanorc

[snazy]

(Recent) nano packages in Ubuntu come with some .nanorc files preinstalled.

jline's NanorcParser sadly fails parsing /usr/share/nano/json.nanorc that comes with Ubuntu's package.

In particular, it fails at this line in /usr/share/nano/json.nanorc:

color brightblue "[][]"

with the exception

java.util.regex.PatternSyntaxException: Unclosed character class near index 3
[][]
   ^
...
	at org.jline.builtins.SyntaxHighlighter$NanorcParser.doPattern(SyntaxHighlighter.java:696)
...

Fixes #1156

[mattirn]

@snazy, I think we should do a more general fix rather than start to resolve patterns one by one.

In java regex characters [ and ] must be always escaped in a bracket expression. In POSIX regex [ character can be added in a bracket expression without escaping. Also ] character can be added without escaping if it is the first character of the bracket expression.

We can add a static method fixRegex(String regex) to Parse class that will escape [ and ] characters in a bracket expression.

[snazy]

Wonder whether this gets into parsing the regex itself. However, do you think its good enough to handle the cases that the Character classes chapter in Pattern's javadoc describes? I.e. looking for [ (unless it's not escaped as \[ and then handle the escaping [ ] ?

[mattirn]

Wonder whether this gets into parsing the regex itself. However, do you think its good enough to handle the cases that the Character classes chapter in Pattern's javadoc describes?

Regexes in nanorc-files follow POSIX standard. Intersections ([a-z&&[def]]), unions ([a-d[m-p]]) etc descriped in Pattern's javadoc are not valid POSIX regexes.

I.e. looking for [ (unless it's not escaped as \[ and then handle the escaping [ ] ?

In regex the first unescaped '[' starts a bracket expression. In POSIX a bracket expression is closed by unescaped ']' if it is not the next character after '[' which started the bracket expression. Every square bracket between the starting and closing brackets must be escaped in order to obtain valid java pattern.