terminal STRING_WITH_TOKENS: 'START' -> 'END';

public class STRING_WITH_TOKENSValueConverter extends DefaultTerminalConverters {
	@Inject StringWithTokens stringWithTokens;
	
	@ValueConverter(rule = "STRING_WITH_TOKENS") //Have also tried the full path: "org.xtext.example.mydsl.SglGrammar.STRING_WITH_TOKENS"
    public IValueConverter<String> STRING_WITH_TOKENS() {
		return stringWithTokens;
	}
        
        private static class StringWithTokens extends AbstractLexerBasedConverter<String> {

			@Override
			public String toValue(String string, INode node) throws ValueConverterException {
				if (string == null) return null;
				return string.substring(5, string.length() - 3);
			}
			
//Don't know if this is correct as well. Worked, when I overwrote the default STRING rule, though.
			@Override
			protected String toEscapedString(String value) {
				return "START" + Strings.convertToJavaString(value, false) + "END";
			}
        	
        }
}

public class SglGrammarRuntimeModule extends AbstractSglGrammarRuntimeModule {
	public Class<? extends STRING_WITH_TOKENSValueConverter> bindSTRINGValueConverter() {
    	return STRING_WITH_TOKENSValueConverter.class;
    }
}