Negative enum values are not recognized

[roge]

Negative enum values are not handled correctly for syntax highlighting.

In this example, a negative enum value is preventing the line from being highlighted correctly:

protoc accepts these enum declarations without issue and they are valid per the language specification:

Enum Values

Enums must contain at least one enum value. To enable a predictive parser implementation and avoid ambiguity, an enum value's name cannot be "option" or "reserved".

EnumValueDecl = EnumValueName equals EnumValueNumber [ CompactOptions ] semicolon .

EnumValueName   = identifier  - ( option | reserved ) .
EnumValueNumber = [ minus ] int_literal .

Examples

JOB_STATE_UNSET = 0;
NEGATIVE = -1;
UNKNOWN = 12 [deprecated = true];