The following xtext grammar will parse your example
grammar org.xtext.example.mydsl.MyDsl with org.eclipse.xtext.common.Terminals
generate myDsl "http://www.xtext.org/example/mydsl/MyDsl"
Model
: lables+=TestLabel+
;
TestLabel returns ResultExpressionRhs
: TestLabelBase ('IS' modifier=AlphabateModifier)?
;
TestLabelBase returns ResultExpressionRhs
: 'VL:' path=STRING
;
AlphabateModifier
: (abc?='ABC' | def?='DEF' | ghi?='GHI')
;
It parses the following test file just fine:
VL:'this is test dsl in xtext' IS ABC
VL:'this is test dsl in xtext' IS DEF
VL:'this is test dsl in xtext' IS GHI
Cheers, Steve