CIdent.pm6

# This file is at C/CIntent.pm6
grammar C::CIdent;
token TOP {^ <ident> $}

# identifier
token ident { <.ident-first> <.ident-rest>* }

# identifier-nondigit
proto token ident-first {*}
token ident-first:sym<under> { '_' }
token ident-first:sym<alpha> { <.alpha> }

# identifier-nondigit | digit
proto token ident-rest {*}
token ident-rest:sym<alpha> { <.ident-first> }
token ident-rest:sym<digit> { <.digit> }

Console

EXPECTED
$ niecza C/CTest.pl6 'test'
｢test｣
 ident => ｢test｣

GOT
$ niecza C/CTest.pl6 'test'
Unhandled exception: System.NullReferenceException: Object reference not set to an instance of an object
  at Niecza.Frame.MakeChild (Niecza.Frame outer, Niecza.SubInfo info, Niecza.P6any sub) [0x00000] in <filename unknown>:0 
  at Niecza.SubInfo.SetupCall (Niecza.Frame caller, Niecza.Frame outer, Niecza.P6any sub, Niecza.Variable[] pos, Niecza.VarHash named, Boolean quiet, Niecza.DispatchEnt de) [0x00000] in <filename unknown>:0 
  at Niecza.P6any.InvokeMethod (Niecza.Frame caller, System.String name, Niecza.Variable[] pos, Niecza.VarHash named) [0x00000] in <filename unknown>:0 
  at C.CIdent.a86e899e-7a30-471a-a6f1-c3dbeba995f4.C3CIdent.ident (Niecza.Frame ) [0x00000] in <filename unknown>:0 
  at Niecza.Kernel.RunCore (Niecza.Frame& cur) [0x00000] in <filename unknown>:0 
  at /Users/ajr/db/local/src/c2drox/lib/C/CIdent.pm6 line 9 (CIdent.ident @ 3) 
  at /Users/ajr/db/local/src/c2drox/lib/C/CIdent.pm6 line 6 (CIdent.TOP @ 3) 
  at /Users/ajr/db/local/src/niecza/lib/CORE.setting line 2990 (Grammar.parse @ 5) 
  at /Users/ajr/db/local/src/c2drox/lib/C/CTest.pl6 line 5 (mainline @ 6) 
  at /Users/ajr/db/local/src/niecza/lib/CORE.setting line 4299 (ANON @ 3) 
  at /Users/ajr/db/local/src/niecza/lib/CORE.setting line 4300 (module-CORE @ 583) 
  at /Users/ajr/db/local/src/niecza/lib/CORE.setting line 4300 (mainline @ 3) 
  at <unknown> line 0 (ExitRunloop @ 0) 

CTest.pl6

use v6;
use C::CIdent;

my $source = @*ARGS[0];
say C::CIdent.parse($source);