Error when evaluating `\\`

[peedromarinhow]

In grammatica 1.6:

INTERNAL ERROR: An internal error in Grammatica has been found.
    Please report this error to the maintainers (see the web
    site for instructions). Be sure to include the Grammatica
    version number, as well as the information below:

net.percederberg.grammatica.GrammarException: token 'LITERAL_CHAR' is invalid, as regular expression contains error(s): Illegal repetition near index 6
'(.|(\\{[+-]?[0-9a-fA-F_]+}))?'
      ^, on line 41
        at net.percederberg.grammatica.Grammar.createTokenizer(Grammar.java:225)
        at net.percederberg.grammatica.Grammatica.debug(Grammatica.java:416)
        at net.percederberg.grammatica.Grammatica.main(Grammatica.java:163)