A query like field_name = '\a' will generate the following notice (and will not work as expected):

Notice: Use of undefined constant digits - assumed 'digits' in /usr/share/tuleap/plugins/tracker/include/Tracker/Report/Query/Advanced/Grammar/Parser.php on line 4734
Call Stack
# Time Memory Function Location
1 0.0001 661504 {main}( ) ../index.php:0
2 0.0452 5103168 TrackerManager->process( ) ../index.php:33
3 0.0459 5143416 TrackerManager->processSubElement( ) ../TrackerManager.class.php:126
4 0.0625 8495736 Tracker_Report->process( ) ../TrackerManager.class.php:109
5 0.0627 8496304 Tracker_Report->parseExpertQuery( ) ../Tracker_Report.class.php:1317
6 0.0627 8496304 Tuleap\Tracker\Report\Query\Advanced\Grammar\Parser->parse( ) ../Tracker_Report.class.php:1658
7 0.0630 8586088 call_user_func ( ) ../Parser.php:4903
8 0.0630 8586088 Tuleap\Tracker\Report\Query\Advanced\Grammar\Parser->peg_parseor_expression( ) ../Parser.php:4903
9 0.0630 8586680 Tuleap\Tracker\Report\Query\Advanced\Grammar\Parser->peg_parseand_expression( ) ../Parser.php:428
10 0.0630 8586776 Tuleap\Tracker\Report\Query\Advanced\Grammar\Parser->peg_parseterm( ) ../Parser.php:532
11 0.0630 8586872 Tuleap\Tracker\Report\Query\Advanced\Grammar\Parser->peg_parseEqualComparison( ) ../Parser.php:637
12 0.0633 8589440 Tuleap\Tracker\Report\Query\Advanced\Grammar\Parser->peg_parseSimpleExpr( ) ../Parser.php:734
13 0.0633 8589536 Tuleap\Tracker\Report\Query\Advanced\Grammar\Parser->peg_parseLiteral( ) ../Parser.php:825
14 0.0633 8589632 Tuleap\Tracker\Report\Query\Advanced\Grammar\Parser->peg_parseString( ) ../Parser.php:924
15 0.0633 8590224 Tuleap\Tracker\Report\Query\Advanced\Grammar\Parser->peg_parseString2( ) ../Parser.php:946
16 0.0634 8591176 Tuleap\Tracker\Report\Query\Advanced\Grammar\Parser->peg_parseescape( ) ../Parser.php:1126
17 0.0634 8591272 Tuleap\Tracker\Report\Query\Advanced\Grammar\Parser->peg_parseunicode( ) ../Parser.php:1372
18 0.0635 8593272 call_user_func ( ) ../Parser.php:1342
19 0.0635 8593304 Tuleap\Tracker\Report\Query\Advanced\Grammar\{closure}( ) ../Parser.php:1342