5.1 and trunk permits you to write e.g. Foo( $array: array ) Bar( this == $array[0] ) but this crashes druing compilation with java.lang.StringIndexOutOfBoundsException: String index out of range: -1 at java.lang.String.substring(String.java:1949) at org.drools.rule.builder.PatternBuilder.buildRestriction(PatternBuilder.java:928) It's OK by this syntax: accessor_element ; ID square_chunk* ; Processing in DescriptorFactory handles []: createAccessorPath(List<BaseDescr> aeList) { ... if (name.indexOf('.') > -1 || name.indexOf('[') > -1) { evaluator = new QualifiedIdentifierRestrictionDescr(); Then in PatternBuilder, we end up in this variant of the overloaded method buildRestriction: private Restriction buildRestriction(final RuleBuildContext context, final InternalReadAccessor extractor, final FieldConstraintDescr fieldConstraintDescr, final QualifiedIdentifierRestrictionDescr qiRestrictionDescr) and this is where it happens, lastDot being == -1. { final int lastDot = qiRestrictionDescr.getText().lastIndexOf( '.' ); final String className = qiRestrictionDescr.getText().substring( 0, lastDot ); If the syntax isn't too permissive, then this method would have to handle x[y] in addition to x.y, etc. -W