1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
/*!
Defines an abstract syntax for regular expressions.
*/

use std::cmp::Ordering;
use std::error;
use std::fmt;

pub use crate::ast::visitor::{visit, Visitor};

pub mod parse;
pub mod print;
mod visitor;

/// An error that occurred while parsing a regular expression into an abstract
/// syntax tree.
///
/// Note that not all ASTs represents a valid regular expression. For example,
/// an AST is constructed without error for `\p{Quux}`, but `Quux` is not a
/// valid Unicode property name. That particular error is reported when
/// translating an AST to the high-level intermediate representation (`HIR`).
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct Error {
    /// The kind of error.
    kind: ErrorKind,
    /// The original pattern that the parser generated the error from. Every
    /// span in an error is a valid range into this string.
    pattern: String,
    /// The span of this error.
    span: Span,
}

impl Error {
    /// Return the type of this error.
    pub fn kind(&self) -> &ErrorKind {
        &self.kind
    }

    /// The original pattern string in which this error occurred.
    ///
    /// Every span reported by this error is reported in terms of this string.
    pub fn pattern(&self) -> &str {
        &self.pattern
    }

    /// Return the span at which this error occurred.
    pub fn span(&self) -> &Span {
        &self.span
    }

    /// Return an auxiliary span. This span exists only for some errors that
    /// benefit from being able to point to two locations in the original
    /// regular expression. For example, "duplicate" errors will have the
    /// main error position set to the duplicate occurrence while its
    /// auxiliary span will be set to the initial occurrence.
    pub fn auxiliary_span(&self) -> Option<&Span> {
        use self::ErrorKind::*;
        match self.kind {
            FlagDuplicate { ref original } => Some(original),
            FlagRepeatedNegation { ref original, .. } => Some(original),
            GroupNameDuplicate { ref original, .. } => Some(original),
            _ => None,
        }
    }
}

/// The type of an error that occurred while building an AST.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum ErrorKind {
    /// The capturing group limit was exceeded.
    ///
    /// Note that this represents a limit on the total number of capturing
    /// groups in a regex and not necessarily the number of nested capturing
    /// groups. That is, the nest limit can be low and it is still possible for
    /// this error to occur.
    CaptureLimitExceeded,
    /// An invalid escape sequence was found in a character class set.
    ClassEscapeInvalid,
    /// An invalid character class range was found. An invalid range is any
    /// range where the start is greater than the end.
    ClassRangeInvalid,
    /// An invalid range boundary was found in a character class. Range
    /// boundaries must be a single literal codepoint, but this error indicates
    /// that something else was found, such as a nested class.
    ClassRangeLiteral,
    /// An opening `[` was found with no corresponding closing `]`.
    ClassUnclosed,
    /// Note that this error variant is no longer used. Namely, a decimal
    /// number can only appear as a repetition quantifier. When the number
    /// in a repetition quantifier is empty, then it gets its own specialized
    /// error, `RepetitionCountDecimalEmpty`.
    DecimalEmpty,
    /// An invalid decimal number was given where one was expected.
    DecimalInvalid,
    /// A bracketed hex literal was empty.
    EscapeHexEmpty,
    /// A bracketed hex literal did not correspond to a Unicode scalar value.
    EscapeHexInvalid,
    /// An invalid hexadecimal digit was found.
    EscapeHexInvalidDigit,
    /// EOF was found before an escape sequence was completed.
    EscapeUnexpectedEof,
    /// An unrecognized escape sequence.
    EscapeUnrecognized,
    /// A dangling negation was used when setting flags, e.g., `i-`.
    FlagDanglingNegation,
    /// A flag was used twice, e.g., `i-i`.
    FlagDuplicate {
        /// The position of the original flag. The error position
        /// points to the duplicate flag.
        original: Span,
    },
    /// The negation operator was used twice, e.g., `-i-s`.
    FlagRepeatedNegation {
        /// The position of the original negation operator. The error position
        /// points to the duplicate negation operator.
        original: Span,
    },
    /// Expected a flag but got EOF, e.g., `(?`.
    FlagUnexpectedEof,
    /// Unrecognized flag, e.g., `a`.
    FlagUnrecognized,
    /// A duplicate capture name was found.
    GroupNameDuplicate {
        /// The position of the initial occurrence of the capture name. The
        /// error position itself points to the duplicate occurrence.
        original: Span,
    },
    /// A capture group name is empty, e.g., `(?P<>abc)`.
    GroupNameEmpty,
    /// An invalid character was seen for a capture group name. This includes
    /// errors where the first character is a digit (even though subsequent
    /// characters are allowed to be digits).
    GroupNameInvalid,
    /// A closing `>` could not be found for a capture group name.
    GroupNameUnexpectedEof,
    /// An unclosed group, e.g., `(ab`.
    ///
    /// The span of this error corresponds to the unclosed parenthesis.
    GroupUnclosed,
    /// An unopened group, e.g., `ab)`.
    GroupUnopened,
    /// The nest limit was exceeded. The limit stored here is the limit
    /// configured in the parser.
    NestLimitExceeded(u32),
    /// The range provided in a counted repetition operator is invalid. The
    /// range is invalid if the start is greater than the end.
    RepetitionCountInvalid,
    /// An opening `{` was not followed by a valid decimal value.
    /// For example, `x{}` or `x{]}` would fail.
    RepetitionCountDecimalEmpty,
    /// An opening `{` was found with no corresponding closing `}`.
    RepetitionCountUnclosed,
    /// A repetition operator was applied to a missing sub-expression. This
    /// occurs, for example, in the regex consisting of just a `*` or even
    /// `(?i)*`. It is, however, possible to create a repetition operating on
    /// an empty sub-expression. For example, `()*` is still considered valid.
    RepetitionMissing,
    /// The Unicode class is not valid. This typically occurs when a `\p` is
    /// followed by something other than a `{`.
    UnicodeClassInvalid,
    /// When octal support is disabled, this error is produced when an octal
    /// escape is used. The octal escape is assumed to be an invocation of
    /// a backreference, which is the common case.
    UnsupportedBackreference,
    /// When syntax similar to PCRE's look-around is used, this error is
    /// returned. Some example syntaxes that are rejected include, but are
    /// not necessarily limited to, `(?=re)`, `(?!re)`, `(?<=re)` and
    /// `(?<!re)`. Note that all of these syntaxes are otherwise invalid; this
    /// error is used to improve the user experience.
    UnsupportedLookAround,
    /// Hints that destructuring should not be exhaustive.
    ///
    /// This enum may grow additional variants, so this makes sure clients
    /// don't count on exhaustive matching. (Otherwise, adding a new variant
    /// could break existing code.)
    #[doc(hidden)]
    __Nonexhaustive,
}

impl error::Error for Error {
    // TODO: Remove this method entirely on the next breaking semver release.
    #[allow(deprecated)]
    fn description(&self) -> &str {
        use self::ErrorKind::*;
        match self.kind {
            CaptureLimitExceeded => "capture group limit exceeded",
            ClassEscapeInvalid => "invalid escape sequence in character class",
            ClassRangeInvalid => "invalid character class range",
            ClassRangeLiteral => "invalid range boundary, must be a literal",
            ClassUnclosed => "unclosed character class",
            DecimalEmpty => "empty decimal literal",
            DecimalInvalid => "invalid decimal literal",
            EscapeHexEmpty => "empty hexadecimal literal",
            EscapeHexInvalid => "invalid hexadecimal literal",
            EscapeHexInvalidDigit => "invalid hexadecimal digit",
            EscapeUnexpectedEof => "unexpected eof (escape sequence)",
            EscapeUnrecognized => "unrecognized escape sequence",
            FlagDanglingNegation => "dangling flag negation operator",
            FlagDuplicate { .. } => "duplicate flag",
            FlagRepeatedNegation { .. } => "repeated negation",
            FlagUnexpectedEof => "unexpected eof (flag)",
            FlagUnrecognized => "unrecognized flag",
            GroupNameDuplicate { .. } => "duplicate capture group name",
            GroupNameEmpty => "empty capture group name",
            GroupNameInvalid => "invalid capture group name",
            GroupNameUnexpectedEof => "unclosed capture group name",
            GroupUnclosed => "unclosed group",
            GroupUnopened => "unopened group",
            NestLimitExceeded(_) => "nest limit exceeded",
            RepetitionCountInvalid => "invalid repetition count range",
            RepetitionCountUnclosed => "unclosed counted repetition",
            RepetitionMissing => "repetition operator missing expression",
            UnicodeClassInvalid => "invalid Unicode character class",
            UnsupportedBackreference => "backreferences are not supported",
            UnsupportedLookAround => "look-around is not supported",
            _ => unreachable!(),
        }
    }
}

impl fmt::Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        crate::error::Formatter::from(self).fmt(f)
    }
}

impl fmt::Display for ErrorKind {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        use self::ErrorKind::*;
        match *self {
            CaptureLimitExceeded => write!(
                f,
                "exceeded the maximum number of \
                 capturing groups ({})",
                ::std::u32::MAX
            ),
            ClassEscapeInvalid => {
                write!(f, "invalid escape sequence found in character class")
            }
            ClassRangeInvalid => write!(
                f,
                "invalid character class range, \
                 the start must be <= the end"
            ),
            ClassRangeLiteral => {
                write!(f, "invalid range boundary, must be a literal")
            }
            ClassUnclosed => write!(f, "unclosed character class"),
            DecimalEmpty => write!(f, "decimal literal empty"),
            DecimalInvalid => write!(f, "decimal literal invalid"),
            EscapeHexEmpty => write!(f, "hexadecimal literal empty"),
            EscapeHexInvalid => {
                write!(f, "hexadecimal literal is not a Unicode scalar value")
            }
            EscapeHexInvalidDigit => write!(f, "invalid hexadecimal digit"),
            EscapeUnexpectedEof => write!(
                f,
                "incomplete escape sequence, \
                 reached end of pattern prematurely"
            ),
            EscapeUnrecognized => write!(f, "unrecognized escape sequence"),
            FlagDanglingNegation => {
                write!(f, "dangling flag negation operator")
            }
            FlagDuplicate { .. } => write!(f, "duplicate flag"),
            FlagRepeatedNegation { .. } => {
                write!(f, "flag negation operator repeated")
            }
            FlagUnexpectedEof => {
                write!(f, "expected flag but got end of regex")
            }
            FlagUnrecognized => write!(f, "unrecognized flag"),
            GroupNameDuplicate { .. } => {
                write!(f, "duplicate capture group name")
            }
            GroupNameEmpty => write!(f, "empty capture group name"),
            GroupNameInvalid => write!(f, "invalid capture group character"),
            GroupNameUnexpectedEof => write!(f, "unclosed capture group name"),
            GroupUnclosed => write!(f, "unclosed group"),
            GroupUnopened => write!(f, "unopened group"),
            NestLimitExceeded(limit) => write!(
                f,
                "exceed the maximum number of \
                 nested parentheses/brackets ({})",
                limit
            ),
            RepetitionCountInvalid => write!(
                f,
                "invalid repetition count range, \
                 the start must be <= the end"
            ),
            RepetitionCountDecimalEmpty => {
                write!(f, "repetition quantifier expects a valid decimal")
            }
            RepetitionCountUnclosed => {
                write!(f, "unclosed counted repetition")
            }
            RepetitionMissing => {
                write!(f, "repetition operator missing expression")
            }
            UnicodeClassInvalid => {
                write!(f, "invalid Unicode character class")
            }
            UnsupportedBackreference => {
                write!(f, "backreferences are not supported")
            }
            UnsupportedLookAround => write!(
                f,
                "look-around, including look-ahead and look-behind, \
                 is not supported"
            ),
            _ => unreachable!(),
        }
    }
}

/// Span represents the position information of a single AST item.
///
/// All span positions are absolute byte offsets that can be used on the
/// original regular expression that was parsed.
#[derive(Clone, Copy, Eq, PartialEq)]
pub struct Span {
    /// The start byte offset.
    pub start: Position,
    /// The end byte offset.
    pub end: Position,
}

impl fmt::Debug for Span {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "Span({:?}, {:?})", self.start, self.end)
    }
}

impl Ord for Span {
    fn cmp(&self, other: &Span) -> Ordering {
        (&self.start, &self.end).cmp(&(&other.start, &other.end))
    }
}

impl PartialOrd for Span {
    fn partial_cmp(&self, other: &Span) -> Option<Ordering> {
        Some(self.cmp(other))
    }
}

/// A single position in a regular expression.
///
/// A position encodes one half of a span, and include the byte offset, line
/// number and column number.
#[derive(Clone, Copy, Eq, PartialEq)]
pub struct Position {
    /// The absolute offset of this position, starting at `0` from the
    /// beginning of the regular expression pattern string.
    pub offset: usize,
    /// The line number, starting at `1`.
    pub line: usize,
    /// The approximate column number, starting at `1`.
    pub column: usize,
}

impl fmt::Debug for Position {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(
            f,
            "Position(o: {:?}, l: {:?}, c: {:?})",
            self.offset, self.line, self.column
        )
    }
}

impl Ord for Position {
    fn cmp(&self, other: &Position) -> Ordering {
        self.offset.cmp(&other.offset)
    }
}

impl PartialOrd for Position {
    fn partial_cmp(&self, other: &Position) -> Option<Ordering> {
        Some(self.cmp(other))
    }
}

impl Span {
    /// Create a new span with the given positions.
    pub fn new(start: Position, end: Position) -> Span {
        Span { start, end }
    }

    /// Create a new span using the given position as the start and end.
    pub fn splat(pos: Position) -> Span {
        Span::new(pos, pos)
    }

    /// Create a new span by replacing the starting the position with the one
    /// given.
    pub fn with_start(self, pos: Position) -> Span {
        Span { start: pos, ..self }
    }

    /// Create a new span by replacing the ending the position with the one
    /// given.
    pub fn with_end(self, pos: Position) -> Span {
        Span { end: pos, ..self }
    }

    /// Returns true if and only if this span occurs on a single line.
    pub fn is_one_line(&self) -> bool {
        self.start.line == self.end.line
    }

    /// Returns true if and only if this span is empty. That is, it points to
    /// a single position in the concrete syntax of a regular expression.
    pub fn is_empty(&self) -> bool {
        self.start.offset == self.end.offset
    }
}

impl Position {
    /// Create a new position with the given information.
    ///
    /// `offset` is the absolute offset of the position, starting at `0` from
    /// the beginning of the regular expression pattern string.
    ///
    /// `line` is the line number, starting at `1`.
    ///
    /// `column` is the approximate column number, starting at `1`.
    pub fn new(offset: usize, line: usize, column: usize) -> Position {
        Position { offset, line, column }
    }
}

/// An abstract syntax tree for a singular expression along with comments
/// found.
///
/// Comments are not stored in the tree itself to avoid complexity. Each
/// comment contains a span of precisely where it occurred in the original
/// regular expression.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct WithComments {
    /// The actual ast.
    pub ast: Ast,
    /// All comments found in the original regular expression.
    pub comments: Vec<Comment>,
}

/// A comment from a regular expression with an associated span.
///
/// A regular expression can only contain comments when the `x` flag is
/// enabled.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct Comment {
    /// The span of this comment, including the beginning `#` and ending `\n`.
    pub span: Span,
    /// The comment text, starting with the first character following the `#`
    /// and ending with the last character preceding the `\n`.
    pub comment: String,
}

/// An abstract syntax tree for a single regular expression.
///
/// An `Ast`'s `fmt::Display` implementation uses constant stack space and heap
/// space proportional to the size of the `Ast`.
///
/// This type defines its own destructor that uses constant stack space and
/// heap space proportional to the size of the `Ast`.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum Ast {
    /// An empty regex that matches everything.
    Empty(Span),
    /// A set of flags, e.g., `(?is)`.
    Flags(SetFlags),
    /// A single character literal, which includes escape sequences.
    Literal(Literal),
    /// The "any character" class.
    Dot(Span),
    /// A single zero-width assertion.
    Assertion(Assertion),
    /// A single character class. This includes all forms of character classes
    /// except for `.`. e.g., `\d`, `\pN`, `[a-z]` and `[[:alpha:]]`.
    Class(Class),
    /// A repetition operator applied to an arbitrary regular expression.
    Repetition(Repetition),
    /// A grouped regular expression.
    Group(Group),
    /// An alternation of regular expressions.
    Alternation(Alternation),
    /// A concatenation of regular expressions.
    Concat(Concat),
}

impl Ast {
    /// Return the span of this abstract syntax tree.
    pub fn span(&self) -> &Span {
        match *self {
            Ast::Empty(ref span) => span,
            Ast::Flags(ref x) => &x.span,
            Ast::Literal(ref x) => &x.span,
            Ast::Dot(ref span) => span,
            Ast::Assertion(ref x) => &x.span,
            Ast::Class(ref x) => x.span(),
            Ast::Repetition(ref x) => &x.span,
            Ast::Group(ref x) => &x.span,
            Ast::Alternation(ref x) => &x.span,
            Ast::Concat(ref x) => &x.span,
        }
    }

    /// Return true if and only if this Ast is empty.
    pub fn is_empty(&self) -> bool {
        match *self {
            Ast::Empty(_) => true,
            _ => false,
        }
    }

    /// Returns true if and only if this AST has any (including possibly empty)
    /// subexpressions.
    fn has_subexprs(&self) -> bool {
        match *self {
            Ast::Empty(_)
            | Ast::Flags(_)
            | Ast::Literal(_)
            | Ast::Dot(_)
            | Ast::Assertion(_) => false,
            Ast::Class(_)
            | Ast::Repetition(_)
            | Ast::Group(_)
            | Ast::Alternation(_)
            | Ast::Concat(_) => true,
        }
    }
}

/// Print a display representation of this Ast.
///
/// This does not preserve any of the original whitespace formatting that may
/// have originally been present in the concrete syntax from which this Ast
/// was generated.
///
/// This implementation uses constant stack space and heap space proportional
/// to the size of the `Ast`.
impl fmt::Display for Ast {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        use crate::ast::print::Printer;
        Printer::new().print(self, f)
    }
}

/// An alternation of regular expressions.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct Alternation {
    /// The span of this alternation.
    pub span: Span,
    /// The alternate regular expressions.
    pub asts: Vec<Ast>,
}

impl Alternation {
    /// Return this alternation as an AST.
    ///
    /// If this alternation contains zero ASTs, then Ast::Empty is
    /// returned. If this alternation contains exactly 1 AST, then the
    /// corresponding AST is returned. Otherwise, Ast::Alternation is returned.
    pub fn into_ast(mut self) -> Ast {
        match self.asts.len() {
            0 => Ast::Empty(self.span),
            1 => self.asts.pop().unwrap(),
            _ => Ast::Alternation(self),
        }
    }
}

/// A concatenation of regular expressions.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct Concat {
    /// The span of this concatenation.
    pub span: Span,
    /// The concatenation regular expressions.
    pub asts: Vec<Ast>,
}

impl Concat {
    /// Return this concatenation as an AST.
    ///
    /// If this concatenation contains zero ASTs, then Ast::Empty is
    /// returned. If this concatenation contains exactly 1 AST, then the
    /// corresponding AST is returned. Otherwise, Ast::Concat is returned.
    pub fn into_ast(mut self) -> Ast {
        match self.asts.len() {
            0 => Ast::Empty(self.span),
            1 => self.asts.pop().unwrap(),
            _ => Ast::Concat(self),
        }
    }
}

/// A single literal expression.
///
/// A literal corresponds to a single Unicode scalar value. Literals may be
/// represented in their literal form, e.g., `a` or in their escaped form,
/// e.g., `\x61`.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct Literal {
    /// The span of this literal.
    pub span: Span,
    /// The kind of this literal.
    pub kind: LiteralKind,
    /// The Unicode scalar value corresponding to this literal.
    pub c: char,
}

impl Literal {
    /// If this literal was written as a `\x` hex escape, then this returns
    /// the corresponding byte value. Otherwise, this returns `None`.
    pub fn byte(&self) -> Option<u8> {
        let short_hex = LiteralKind::HexFixed(HexLiteralKind::X);
        if self.c as u32 <= 255 && self.kind == short_hex {
            Some(self.c as u8)
        } else {
            None
        }
    }
}

/// The kind of a single literal expression.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum LiteralKind {
    /// The literal is written verbatim, e.g., `a` or `☃`.
    Verbatim,
    /// The literal is written as an escape because it is punctuation, e.g.,
    /// `\*` or `\[`.
    Punctuation,
    /// The literal is written as an octal escape, e.g., `\141`.
    Octal,
    /// The literal is written as a hex code with a fixed number of digits
    /// depending on the type of the escape, e.g., `\x61` or or `\u0061` or
    /// `\U00000061`.
    HexFixed(HexLiteralKind),
    /// The literal is written as a hex code with a bracketed number of
    /// digits. The only restriction is that the bracketed hex code must refer
    /// to a valid Unicode scalar value.
    HexBrace(HexLiteralKind),
    /// The literal is written as a specially recognized escape, e.g., `\f`
    /// or `\n`.
    Special(SpecialLiteralKind),
}

/// The type of a special literal.
///
/// A special literal is a special escape sequence recognized by the regex
/// parser, e.g., `\f` or `\n`.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum SpecialLiteralKind {
    /// Bell, spelled `\a` (`\x07`).
    Bell,
    /// Form feed, spelled `\f` (`\x0C`).
    FormFeed,
    /// Tab, spelled `\t` (`\x09`).
    Tab,
    /// Line feed, spelled `\n` (`\x0A`).
    LineFeed,
    /// Carriage return, spelled `\r` (`\x0D`).
    CarriageReturn,
    /// Vertical tab, spelled `\v` (`\x0B`).
    VerticalTab,
    /// Space, spelled `\ ` (`\x20`). Note that this can only appear when
    /// parsing in verbose mode.
    Space,
}

/// The type of a Unicode hex literal.
///
/// Note that all variants behave the same when used with brackets. They only
/// differ when used without brackets in the number of hex digits that must
/// follow.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum HexLiteralKind {
    /// A `\x` prefix. When used without brackets, this form is limited to
    /// two digits.
    X,
    /// A `\u` prefix. When used without brackets, this form is limited to
    /// four digits.
    UnicodeShort,
    /// A `\U` prefix. When used without brackets, this form is limited to
    /// eight digits.
    UnicodeLong,
}

impl HexLiteralKind {
    /// The number of digits that must be used with this literal form when
    /// used without brackets. When used with brackets, there is no
    /// restriction on the number of digits.
    pub fn digits(&self) -> u32 {
        match *self {
            HexLiteralKind::X => 2,
            HexLiteralKind::UnicodeShort => 4,
            HexLiteralKind::UnicodeLong => 8,
        }
    }
}

/// A single character class expression.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum Class {
    /// A Unicode character class, e.g., `\pL` or `\p{Greek}`.
    Unicode(ClassUnicode),
    /// A perl character class, e.g., `\d` or `\W`.
    Perl(ClassPerl),
    /// A bracketed character class set, which may contain zero or more
    /// character ranges and/or zero or more nested classes. e.g.,
    /// `[a-zA-Z\pL]`.
    Bracketed(ClassBracketed),
}

impl Class {
    /// Return the span of this character class.
    pub fn span(&self) -> &Span {
        match *self {
            Class::Perl(ref x) => &x.span,
            Class::Unicode(ref x) => &x.span,
            Class::Bracketed(ref x) => &x.span,
        }
    }
}

/// A Perl character class.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct ClassPerl {
    /// The span of this class.
    pub span: Span,
    /// The kind of Perl class.
    pub kind: ClassPerlKind,
    /// Whether the class is negated or not. e.g., `\d` is not negated but
    /// `\D` is.
    pub negated: bool,
}

/// The available Perl character classes.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum ClassPerlKind {
    /// Decimal numbers.
    Digit,
    /// Whitespace.
    Space,
    /// Word characters.
    Word,
}

/// An ASCII character class.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct ClassAscii {
    /// The span of this class.
    pub span: Span,
    /// The kind of ASCII class.
    pub kind: ClassAsciiKind,
    /// Whether the class is negated or not. e.g., `[[:alpha:]]` is not negated
    /// but `[[:^alpha:]]` is.
    pub negated: bool,
}

/// The available ASCII character classes.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum ClassAsciiKind {
    /// `[0-9A-Za-z]`
    Alnum,
    /// `[A-Za-z]`
    Alpha,
    /// `[\x00-\x7F]`
    Ascii,
    /// `[ \t]`
    Blank,
    /// `[\x00-\x1F\x7F]`
    Cntrl,
    /// `[0-9]`
    Digit,
    /// `[!-~]`
    Graph,
    /// `[a-z]`
    Lower,
    /// `[ -~]`
    Print,
    /// `[!-/:-@\[-`{-~]`
    Punct,
    /// `[\t\n\v\f\r ]`
    Space,
    /// `[A-Z]`
    Upper,
    /// `[0-9A-Za-z_]`
    Word,
    /// `[0-9A-Fa-f]`
    Xdigit,
}

impl ClassAsciiKind {
    /// Return the corresponding ClassAsciiKind variant for the given name.
    ///
    /// The name given should correspond to the lowercase version of the
    /// variant name. e.g., `cntrl` is the name for `ClassAsciiKind::Cntrl`.
    ///
    /// If no variant with the corresponding name exists, then `None` is
    /// returned.
    pub fn from_name(name: &str) -> Option<ClassAsciiKind> {
        use self::ClassAsciiKind::*;
        match name {
            "alnum" => Some(Alnum),
            "alpha" => Some(Alpha),
            "ascii" => Some(Ascii),
            "blank" => Some(Blank),
            "cntrl" => Some(Cntrl),
            "digit" => Some(Digit),
            "graph" => Some(Graph),
            "lower" => Some(Lower),
            "print" => Some(Print),
            "punct" => Some(Punct),
            "space" => Some(Space),
            "upper" => Some(Upper),
            "word" => Some(Word),
            "xdigit" => Some(Xdigit),
            _ => None,
        }
    }
}

/// A Unicode character class.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct ClassUnicode {
    /// The span of this class.
    pub span: Span,
    /// Whether this class is negated or not.
    ///
    /// Note: be careful when using this attribute. This specifically refers
    /// to whether the class is written as `\p` or `\P`, where the latter
    /// is `negated = true`. However, it also possible to write something like
    /// `\P{scx!=Katakana}` which is actually equivalent to
    /// `\p{scx=Katakana}` and is therefore not actually negated even though
    /// `negated = true` here. To test whether this class is truly negated
    /// or not, use the `is_negated` method.
    pub negated: bool,
    /// The kind of Unicode class.
    pub kind: ClassUnicodeKind,
}

impl ClassUnicode {
    /// Returns true if this class has been negated.
    ///
    /// Note that this takes the Unicode op into account, if it's present.
    /// e.g., `is_negated` for `\P{scx!=Katakana}` will return `false`.
    pub fn is_negated(&self) -> bool {
        match self.kind {
            ClassUnicodeKind::NamedValue {
                op: ClassUnicodeOpKind::NotEqual,
                ..
            } => !self.negated,
            _ => self.negated,
        }
    }
}

/// The available forms of Unicode character classes.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum ClassUnicodeKind {
    /// A one letter abbreviated class, e.g., `\pN`.
    OneLetter(char),
    /// A binary property, general category or script. The string may be
    /// empty.
    Named(String),
    /// A property name and an associated value.
    NamedValue {
        /// The type of Unicode op used to associate `name` with `value`.
        op: ClassUnicodeOpKind,
        /// The property name (which may be empty).
        name: String,
        /// The property value (which may be empty).
        value: String,
    },
}

/// The type of op used in a Unicode character class.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum ClassUnicodeOpKind {
    /// A property set to a specific value, e.g., `\p{scx=Katakana}`.
    Equal,
    /// A property set to a specific value using a colon, e.g.,
    /// `\p{scx:Katakana}`.
    Colon,
    /// A property that isn't a particular value, e.g., `\p{scx!=Katakana}`.
    NotEqual,
}

impl ClassUnicodeOpKind {
    /// Whether the op is an equality op or not.
    pub fn is_equal(&self) -> bool {
        match *self {
            ClassUnicodeOpKind::Equal | ClassUnicodeOpKind::Colon => true,
            _ => false,
        }
    }
}

/// A bracketed character class, e.g., `[a-z0-9]`.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct ClassBracketed {
    /// The span of this class.
    pub span: Span,
    /// Whether this class is negated or not. e.g., `[a]` is not negated but
    /// `[^a]` is.
    pub negated: bool,
    /// The type of this set. A set is either a normal union of things, e.g.,
    /// `[abc]` or a result of applying set operations, e.g., `[\pL--c]`.
    pub kind: ClassSet,
}

/// A character class set.
///
/// This type corresponds to the internal structure of a bracketed character
/// class. That is, every bracketed character is one of two types: a union of
/// items (literals, ranges, other bracketed classes) or a tree of binary set
/// operations.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum ClassSet {
    /// An item, which can be a single literal, range, nested character class
    /// or a union of items.
    Item(ClassSetItem),
    /// A single binary operation (i.e., &&, -- or ~~).
    BinaryOp(ClassSetBinaryOp),
}

impl ClassSet {
    /// Build a set from a union.
    pub fn union(ast: ClassSetUnion) -> ClassSet {
        ClassSet::Item(ClassSetItem::Union(ast))
    }

    /// Return the span of this character class set.
    pub fn span(&self) -> &Span {
        match *self {
            ClassSet::Item(ref x) => x.span(),
            ClassSet::BinaryOp(ref x) => &x.span,
        }
    }

    /// Return true if and only if this class set is empty.
    fn is_empty(&self) -> bool {
        match *self {
            ClassSet::Item(ClassSetItem::Empty(_)) => true,
            _ => false,
        }
    }
}

/// A single component of a character class set.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum ClassSetItem {
    /// An empty item.
    ///
    /// Note that a bracketed character class cannot contain a single empty
    /// item. Empty items can appear when using one of the binary operators.
    /// For example, `[&&]` is the intersection of two empty classes.
    Empty(Span),
    /// A single literal.
    Literal(Literal),
    /// A range between two literals.
    Range(ClassSetRange),
    /// An ASCII character class, e.g., `[:alnum:]` or `[:punct:]`.
    Ascii(ClassAscii),
    /// A Unicode character class, e.g., `\pL` or `\p{Greek}`.
    Unicode(ClassUnicode),
    /// A perl character class, e.g., `\d` or `\W`.
    Perl(ClassPerl),
    /// A bracketed character class set, which may contain zero or more
    /// character ranges and/or zero or more nested classes. e.g.,
    /// `[a-zA-Z\pL]`.
    Bracketed(Box<ClassBracketed>),
    /// A union of items.
    Union(ClassSetUnion),
}

impl ClassSetItem {
    /// Return the span of this character class set item.
    pub fn span(&self) -> &Span {
        match *self {
            ClassSetItem::Empty(ref span) => span,
            ClassSetItem::Literal(ref x) => &x.span,
            ClassSetItem::Range(ref x) => &x.span,
            ClassSetItem::Ascii(ref x) => &x.span,
            ClassSetItem::Perl(ref x) => &x.span,
            ClassSetItem::Unicode(ref x) => &x.span,
            ClassSetItem::Bracketed(ref x) => &x.span,
            ClassSetItem::Union(ref x) => &x.span,
        }
    }
}

/// A single character class range in a set.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct ClassSetRange {
    /// The span of this range.
    pub span: Span,
    /// The start of this range.
    pub start: Literal,
    /// The end of this range.
    pub end: Literal,
}

impl ClassSetRange {
    /// Returns true if and only if this character class range is valid.
    ///
    /// The only case where a range is invalid is if its start is greater than
    /// its end.
    pub fn is_valid(&self) -> bool {
        self.start.c <= self.end.c
    }
}

/// A union of items inside a character class set.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct ClassSetUnion {
    /// The span of the items in this operation. e.g., the `a-z0-9` in
    /// `[^a-z0-9]`
    pub span: Span,
    /// The sequence of items that make up this union.
    pub items: Vec<ClassSetItem>,
}

impl ClassSetUnion {
    /// Push a new item in this union.
    ///
    /// The ending position of this union's span is updated to the ending
    /// position of the span of the item given. If the union is empty, then
    /// the starting position of this union is set to the starting position
    /// of this item.
    ///
    /// In other words, if you only use this method to add items to a union
    /// and you set the spans on each item correctly, then you should never
    /// need to adjust the span of the union directly.
    pub fn push(&mut self, item: ClassSetItem) {
        if self.items.is_empty() {
            self.span.start = item.span().start;
        }
        self.span.end = item.span().end;
        self.items.push(item);
    }

    /// Return this union as a character class set item.
    ///
    /// If this union contains zero items, then an empty union is
    /// returned. If this concatenation contains exactly 1 item, then the
    /// corresponding item is returned. Otherwise, ClassSetItem::Union is
    /// returned.
    pub fn into_item(mut self) -> ClassSetItem {
        match self.items.len() {
            0 => ClassSetItem::Empty(self.span),
            1 => self.items.pop().unwrap(),
            _ => ClassSetItem::Union(self),
        }
    }
}

/// A Unicode character class set operation.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct ClassSetBinaryOp {
    /// The span of this operation. e.g., the `a-z--[h-p]` in `[a-z--h-p]`.
    pub span: Span,
    /// The type of this set operation.
    pub kind: ClassSetBinaryOpKind,
    /// The left hand side of the operation.
    pub lhs: Box<ClassSet>,
    /// The right hand side of the operation.
    pub rhs: Box<ClassSet>,
}

/// The type of a Unicode character class set operation.
///
/// Note that this doesn't explicitly represent union since there is no
/// explicit union operator. Concatenation inside a character class corresponds
/// to the union operation.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum ClassSetBinaryOpKind {
    /// The intersection of two sets, e.g., `\pN&&[a-z]`.
    Intersection,
    /// The difference of two sets, e.g., `\pN--[0-9]`.
    Difference,
    /// The symmetric difference of two sets. The symmetric difference is the
    /// set of elements belonging to one but not both sets.
    /// e.g., `[\pL~~[:ascii:]]`.
    SymmetricDifference,
}

/// A single zero-width assertion.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct Assertion {
    /// The span of this assertion.
    pub span: Span,
    /// The assertion kind, e.g., `\b` or `^`.
    pub kind: AssertionKind,
}

/// An assertion kind.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum AssertionKind {
    /// `^`
    StartLine,
    /// `$`
    EndLine,
    /// `\A`
    StartText,
    /// `\z`
    EndText,
    /// `\b`
    WordBoundary,
    /// `\B`
    NotWordBoundary,
}

/// A repetition operation applied to a regular expression.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct Repetition {
    /// The span of this operation.
    pub span: Span,
    /// The actual operation.
    pub op: RepetitionOp,
    /// Whether this operation was applied greedily or not.
    pub greedy: bool,
    /// The regular expression under repetition.
    pub ast: Box<Ast>,
}

/// The repetition operator itself.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct RepetitionOp {
    /// The span of this operator. This includes things like `+`, `*?` and
    /// `{m,n}`.
    pub span: Span,
    /// The type of operation.
    pub kind: RepetitionKind,
}

/// The kind of a repetition operator.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum RepetitionKind {
    /// `?`
    ZeroOrOne,
    /// `*`
    ZeroOrMore,
    /// `+`
    OneOrMore,
    /// `{m,n}`
    Range(RepetitionRange),
}

/// A range repetition operator.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum RepetitionRange {
    /// `{m}`
    Exactly(u32),
    /// `{m,}`
    AtLeast(u32),
    /// `{m,n}`
    Bounded(u32, u32),
}

impl RepetitionRange {
    /// Returns true if and only if this repetition range is valid.
    ///
    /// The only case where a repetition range is invalid is if it is bounded
    /// and its start is greater than its end.
    pub fn is_valid(&self) -> bool {
        match *self {
            RepetitionRange::Bounded(s, e) if s > e => false,
            _ => true,
        }
    }
}

/// A grouped regular expression.
///
/// This includes both capturing and non-capturing groups. This does **not**
/// include flag-only groups like `(?is)`, but does contain any group that
/// contains a sub-expression, e.g., `(a)`, `(?P<name>a)`, `(?:a)` and
/// `(?is:a)`.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct Group {
    /// The span of this group.
    pub span: Span,
    /// The kind of this group.
    pub kind: GroupKind,
    /// The regular expression in this group.
    pub ast: Box<Ast>,
}

impl Group {
    /// If this group is non-capturing, then this returns the (possibly empty)
    /// set of flags. Otherwise, `None` is returned.
    pub fn flags(&self) -> Option<&Flags> {
        match self.kind {
            GroupKind::NonCapturing(ref flags) => Some(flags),
            _ => None,
        }
    }

    /// Returns true if and only if this group is capturing.
    pub fn is_capturing(&self) -> bool {
        match self.kind {
            GroupKind::CaptureIndex(_) | GroupKind::CaptureName(_) => true,
            GroupKind::NonCapturing(_) => false,
        }
    }

    /// Returns the capture index of this group, if this is a capturing group.
    ///
    /// This returns a capture index precisely when `is_capturing` is `true`.
    pub fn capture_index(&self) -> Option<u32> {
        match self.kind {
            GroupKind::CaptureIndex(i) => Some(i),
            GroupKind::CaptureName(ref x) => Some(x.index),
            GroupKind::NonCapturing(_) => None,
        }
    }
}

/// The kind of a group.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum GroupKind {
    /// `(a)`
    CaptureIndex(u32),
    /// `(?P<name>a)`
    CaptureName(CaptureName),
    /// `(?:a)` and `(?i:a)`
    NonCapturing(Flags),
}

/// A capture name.
///
/// This corresponds to the name itself between the angle brackets in, e.g.,
/// `(?P<foo>expr)`.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct CaptureName {
    /// The span of this capture name.
    pub span: Span,
    /// The capture name.
    pub name: String,
    /// The capture index.
    pub index: u32,
}

/// A group of flags that is not applied to a particular regular expression.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct SetFlags {
    /// The span of these flags, including the grouping parentheses.
    pub span: Span,
    /// The actual sequence of flags.
    pub flags: Flags,
}

/// A group of flags.
///
/// This corresponds only to the sequence of flags themselves, e.g., `is-u`.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct Flags {
    /// The span of this group of flags.
    pub span: Span,
    /// A sequence of flag items. Each item is either a flag or a negation
    /// operator.
    pub items: Vec<FlagsItem>,
}

impl Flags {
    /// Add the given item to this sequence of flags.
    ///
    /// If the item was added successfully, then `None` is returned. If the
    /// given item is a duplicate, then `Some(i)` is returned, where
    /// `items[i].kind == item.kind`.
    pub fn add_item(&mut self, item: FlagsItem) -> Option<usize> {
        for (i, x) in self.items.iter().enumerate() {
            if x.kind == item.kind {
                return Some(i);
            }
        }
        self.items.push(item);
        None
    }

    /// Returns the state of the given flag in this set.
    ///
    /// If the given flag is in the set but is negated, then `Some(false)` is
    /// returned.
    ///
    /// If the given flag is in the set and is not negated, then `Some(true)`
    /// is returned.
    ///
    /// Otherwise, `None` is returned.
    pub fn flag_state(&self, flag: Flag) -> Option<bool> {
        let mut negated = false;
        for x in &self.items {
            match x.kind {
                FlagsItemKind::Negation => {
                    negated = true;
                }
                FlagsItemKind::Flag(ref xflag) if xflag == &flag => {
                    return Some(!negated);
                }
                _ => {}
            }
        }
        None
    }
}

/// A single item in a group of flags.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct FlagsItem {
    /// The span of this item.
    pub span: Span,
    /// The kind of this item.
    pub kind: FlagsItemKind,
}

/// The kind of an item in a group of flags.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum FlagsItemKind {
    /// A negation operator applied to all subsequent flags in the enclosing
    /// group.
    Negation,
    /// A single flag in a group.
    Flag(Flag),
}

impl FlagsItemKind {
    /// Returns true if and only if this item is a negation operator.
    pub fn is_negation(&self) -> bool {
        match *self {
            FlagsItemKind::Negation => true,
            _ => false,
        }
    }
}

/// A single flag.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum Flag {
    /// `i`
    CaseInsensitive,
    /// `m`
    MultiLine,
    /// `s`
    DotMatchesNewLine,
    /// `U`
    SwapGreed,
    /// `u`
    Unicode,
    /// `x`
    IgnoreWhitespace,
}

/// A custom `Drop` impl is used for `Ast` such that it uses constant stack
/// space but heap space proportional to the depth of the `Ast`.
impl Drop for Ast {
    fn drop(&mut self) {
        use std::mem;

        match *self {
            Ast::Empty(_)
            | Ast::Flags(_)
            | Ast::Literal(_)
            | Ast::Dot(_)
            | Ast::Assertion(_)
            // Classes are recursive, so they get their own Drop impl.
            | Ast::Class(_) => return,
            Ast::Repetition(ref x) if !x.ast.has_subexprs() => return,
            Ast::Group(ref x) if !x.ast.has_subexprs() => return,
            Ast::Alternation(ref x) if x.asts.is_empty() => return,
            Ast::Concat(ref x) if x.asts.is_empty() => return,
            _ => {}
        }

        let empty_span = || Span::splat(Position::new(0, 0, 0));
        let empty_ast = || Ast::Empty(empty_span());
        let mut stack = vec![mem::replace(self, empty_ast())];
        while let Some(mut ast) = stack.pop() {
            match ast {
                Ast::Empty(_)
                | Ast::Flags(_)
                | Ast::Literal(_)
                | Ast::Dot(_)
                | Ast::Assertion(_)
                // Classes are recursive, so they get their own Drop impl.
                | Ast::Class(_) => {}
                Ast::Repetition(ref mut x) => {
                    stack.push(mem::replace(&mut x.ast, empty_ast()));
                }
                Ast::Group(ref mut x) => {
                    stack.push(mem::replace(&mut x.ast, empty_ast()));
                }
                Ast::Alternation(ref mut x) => {
                    stack.extend(x.asts.drain(..));
                }
                Ast::Concat(ref mut x) => {
                    stack.extend(x.asts.drain(..));
                }
            }
        }
    }
}

/// A custom `Drop` impl is used for `ClassSet` such that it uses constant
/// stack space but heap space proportional to the depth of the `ClassSet`.
impl Drop for ClassSet {
    fn drop(&mut self) {
        use std::mem;

        match *self {
            ClassSet::Item(ref item) => match *item {
                ClassSetItem::Empty(_)
                | ClassSetItem::Literal(_)
                | ClassSetItem::Range(_)
                | ClassSetItem::Ascii(_)
                | ClassSetItem::Unicode(_)
                | ClassSetItem::Perl(_) => return,
                ClassSetItem::Bracketed(ref x) => {
                    if x.kind.is_empty() {
                        return;
                    }
                }
                ClassSetItem::Union(ref x) => {
                    if x.items.is_empty() {
                        return;
                    }
                }
            },
            ClassSet::BinaryOp(ref op) => {
                if op.lhs.is_empty() && op.rhs.is_empty() {
                    return;
                }
            }
        }

        let empty_span = || Span::splat(Position::new(0, 0, 0));
        let empty_set = || ClassSet::Item(ClassSetItem::Empty(empty_span()));
        let mut stack = vec![mem::replace(self, empty_set())];
        while let Some(mut set) = stack.pop() {
            match set {
                ClassSet::Item(ref mut item) => match *item {
                    ClassSetItem::Empty(_)
                    | ClassSetItem::Literal(_)
                    | ClassSetItem::Range(_)
                    | ClassSetItem::Ascii(_)
                    | ClassSetItem::Unicode(_)
                    | ClassSetItem::Perl(_) => {}
                    ClassSetItem::Bracketed(ref mut x) => {
                        stack.push(mem::replace(&mut x.kind, empty_set()));
                    }
                    ClassSetItem::Union(ref mut x) => {
                        stack.extend(x.items.drain(..).map(ClassSet::Item));
                    }
                },
                ClassSet::BinaryOp(ref mut op) => {
                    stack.push(mem::replace(&mut op.lhs, empty_set()));
                    stack.push(mem::replace(&mut op.rhs, empty_set()));
                }
            }
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    // We use a thread with an explicit stack size to test that our destructor
    // for Ast can handle arbitrarily sized expressions in constant stack
    // space. In case we run on a platform without threads (WASM?), we limit
    // this test to Windows/Unix.
    #[test]
    #[cfg(any(unix, windows))]
    fn no_stack_overflow_on_drop() {
        use std::thread;

        let run = || {
            let span = || Span::splat(Position::new(0, 0, 0));
            let mut ast = Ast::Empty(span());
            for i in 0..200 {
                ast = Ast::Group(Group {
                    span: span(),
                    kind: GroupKind::CaptureIndex(i),
                    ast: Box::new(ast),
                });
            }
            assert!(!ast.is_empty());
        };

        // We run our test on a thread with a small stack size so we can
        // force the issue more easily.
        //
        // NOTE(2023-03-21): It turns out that some platforms (like FreeBSD)
        // will just barf with very small stack sizes. So we bump this up a bit
        // to give more room to breath. When I did this, I confirmed that if
        // I remove the custom `Drop` impl for `Ast`, then this test does
        // indeed still fail with a stack overflow. (At the time of writing, I
        // had to bump it all the way up to 32K before the test would pass even
        // without the custom `Drop` impl. So 16K seems like a safe number
        // here.)
        //
        // See: https://github.com/rust-lang/regex/issues/967
        thread::Builder::new()
            .stack_size(16 << 10)
            .spawn(run)
            .unwrap()
            .join()
            .unwrap();
    }
}