RegExp: Nested subtractions are translated wrongly to Alex

[andreasabel]

LBNF does not support nested subtraction in regexps, like in

token Name (char - [ "(){};.@\"" ] - [ " \n\t" ]) + ;

It works with parentheses:

token Name ((char - [ "(){};.@\"" ]) - [ " \n\t" ]) + ;

This is translated to Alex as

(($u # [\( \) \{ \} \; \. \@ \"]) # [\  \n \t]) +

which is rejected by Alex. However, Alex supports it without parentheses:

($u # [\( \) \{ \} \; \. \@ \"] # [\  \n \t]) +

Both BNFC and Alex are behaving silly here, I must say.

[andreasabel]

Fixes:

• BNFC now accepts left-nested subtraction without parentheses.
• Regex are simplified before passed to the backends.