regex_automata/util/
look.rs

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
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
/*!
Types and routines for working with look-around assertions.

This module principally defines two types:

* [`Look`] enumerates all of the assertions supported by this crate.
* [`LookSet`] provides a way to efficiently store a set of [`Look`] values.
* [`LookMatcher`] provides routines for checking whether a `Look` or a
`LookSet` matches at a particular position in a haystack.
*/

// LAMENTATION: Sadly, a lot of the API of `Look` and `LookSet` were basically
// copied verbatim from the regex-syntax crate. I would have no problems using
// the regex-syntax types and defining the matching routines (only found
// in this crate) as free functions, except the `Look` and `LookSet` types
// are used in lots of places. Including in places we expect to work when
// regex-syntax is *not* enabled, such as in the definition of the NFA itself.
//
// Thankfully the code we copy is pretty simple and there isn't much of it.
// Otherwise, the rest of this module deals with *matching* the assertions,
// which is not something that regex-syntax handles.

use crate::util::{escape::DebugByte, utf8};

/// A look-around assertion.
///
/// An assertion matches at a position between characters in a haystack.
/// Namely, it does not actually "consume" any input as most parts of a regular
/// expression do. Assertions are a way of stating that some property must be
/// true at a particular point during matching.
///
/// For example, `(?m)^[a-z]+$` is a pattern that:
///
/// * Scans the haystack for a position at which `(?m:^)` is satisfied. That
/// occurs at either the beginning of the haystack, or immediately following
/// a `\n` character.
/// * Looks for one or more occurrences of `[a-z]`.
/// * Once `[a-z]+` has matched as much as it can, an overall match is only
/// reported when `[a-z]+` stops just before a `\n`.
///
/// So in this case, `abc` and `\nabc\n` match, but `\nabc1\n` does not.
///
/// Assertions are also called "look-around," "look-behind" and "look-ahead."
/// Specifically, some assertions are look-behind (like `^`), other assertions
/// are look-ahead (like `$`) and yet other assertions are both look-ahead and
/// look-behind (like `\b`).
///
/// # Assertions in an NFA
///
/// An assertion in a [`thompson::NFA`](crate::nfa::thompson::NFA) can be
/// thought of as a conditional epsilon transition. That is, a matching engine
/// like the [`PikeVM`](crate::nfa::thompson::pikevm::PikeVM) only permits
/// moving through conditional epsilon transitions when their condition
/// is satisfied at whatever position the `PikeVM` is currently at in the
/// haystack.
///
/// How assertions are handled in a `DFA` is trickier, since a DFA does not
/// have epsilon transitions at all. In this case, they are compiled into the
/// automaton itself, at the expense of more states than what would be required
/// without an assertion.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum Look {
    /// Match the beginning of text. Specifically, this matches at the starting
    /// position of the input.
    Start = 1 << 0,
    /// Match the end of text. Specifically, this matches at the ending
    /// position of the input.
    End = 1 << 1,
    /// Match the beginning of a line or the beginning of text. Specifically,
    /// this matches at the starting position of the input, or at the position
    /// immediately following a `\n` character.
    StartLF = 1 << 2,
    /// Match the end of a line or the end of text. Specifically, this matches
    /// at the end position of the input, or at the position immediately
    /// preceding a `\n` character.
    EndLF = 1 << 3,
    /// Match the beginning of a line or the beginning of text. Specifically,
    /// this matches at the starting position of the input, or at the position
    /// immediately following either a `\r` or `\n` character, but never after
    /// a `\r` when a `\n` follows.
    StartCRLF = 1 << 4,
    /// Match the end of a line or the end of text. Specifically, this matches
    /// at the end position of the input, or at the position immediately
    /// preceding a `\r` or `\n` character, but never before a `\n` when a `\r`
    /// precedes it.
    EndCRLF = 1 << 5,
    /// Match an ASCII-only word boundary. That is, this matches a position
    /// where the left adjacent character and right adjacent character
    /// correspond to a word and non-word or a non-word and word character.
    WordAscii = 1 << 6,
    /// Match an ASCII-only negation of a word boundary.
    WordAsciiNegate = 1 << 7,
    /// Match a Unicode-aware word boundary. That is, this matches a position
    /// where the left adjacent character and right adjacent character
    /// correspond to a word and non-word or a non-word and word character.
    WordUnicode = 1 << 8,
    /// Match a Unicode-aware negation of a word boundary.
    WordUnicodeNegate = 1 << 9,
    /// Match the start of an ASCII-only word boundary. That is, this matches a
    /// position at either the beginning of the haystack or where the previous
    /// character is not a word character and the following character is a word
    /// character.
    WordStartAscii = 1 << 10,
    /// Match the end of an ASCII-only word boundary. That is, this matches
    /// a position at either the end of the haystack or where the previous
    /// character is a word character and the following character is not a word
    /// character.
    WordEndAscii = 1 << 11,
    /// Match the start of a Unicode word boundary. That is, this matches a
    /// position at either the beginning of the haystack or where the previous
    /// character is not a word character and the following character is a word
    /// character.
    WordStartUnicode = 1 << 12,
    /// Match the end of a Unicode word boundary. That is, this matches a
    /// position at either the end of the haystack or where the previous
    /// character is a word character and the following character is not a word
    /// character.
    WordEndUnicode = 1 << 13,
    /// Match the start half of an ASCII-only word boundary. That is, this
    /// matches a position at either the beginning of the haystack or where the
    /// previous character is not a word character.
    WordStartHalfAscii = 1 << 14,
    /// Match the end half of an ASCII-only word boundary. That is, this
    /// matches a position at either the end of the haystack or where the
    /// following character is not a word character.
    WordEndHalfAscii = 1 << 15,
    /// Match the start half of a Unicode word boundary. That is, this matches
    /// a position at either the beginning of the haystack or where the
    /// previous character is not a word character.
    WordStartHalfUnicode = 1 << 16,
    /// Match the end half of a Unicode word boundary. That is, this matches
    /// a position at either the end of the haystack or where the following
    /// character is not a word character.
    WordEndHalfUnicode = 1 << 17,
}

impl Look {
    /// Flip the look-around assertion to its equivalent for reverse searches.
    /// For example, `StartLF` gets translated to `EndLF`.
    ///
    /// Some assertions, such as `WordUnicode`, remain the same since they
    /// match the same positions regardless of the direction of the search.
    #[inline]
    pub const fn reversed(self) -> Look {
        match self {
            Look::Start => Look::End,
            Look::End => Look::Start,
            Look::StartLF => Look::EndLF,
            Look::EndLF => Look::StartLF,
            Look::StartCRLF => Look::EndCRLF,
            Look::EndCRLF => Look::StartCRLF,
            Look::WordAscii => Look::WordAscii,
            Look::WordAsciiNegate => Look::WordAsciiNegate,
            Look::WordUnicode => Look::WordUnicode,
            Look::WordUnicodeNegate => Look::WordUnicodeNegate,
            Look::WordStartAscii => Look::WordEndAscii,
            Look::WordEndAscii => Look::WordStartAscii,
            Look::WordStartUnicode => Look::WordEndUnicode,
            Look::WordEndUnicode => Look::WordStartUnicode,
            Look::WordStartHalfAscii => Look::WordEndHalfAscii,
            Look::WordEndHalfAscii => Look::WordStartHalfAscii,
            Look::WordStartHalfUnicode => Look::WordEndHalfUnicode,
            Look::WordEndHalfUnicode => Look::WordStartHalfUnicode,
        }
    }

    /// Return the underlying representation of this look-around enumeration
    /// as an integer. Giving the return value to the [`Look::from_repr`]
    /// constructor is guaranteed to return the same look-around variant that
    /// one started with within a semver compatible release of this crate.
    #[inline]
    pub const fn as_repr(self) -> u32 {
        // AFAIK, 'as' is the only way to zero-cost convert an int enum to an
        // actual int.
        self as u32
    }

    /// Given the underlying representation of a `Look` value, return the
    /// corresponding `Look` value if the representation is valid. Otherwise
    /// `None` is returned.
    #[inline]
    pub const fn from_repr(repr: u32) -> Option<Look> {
        match repr {
            0b00_0000_0000_0000_0001 => Some(Look::Start),
            0b00_0000_0000_0000_0010 => Some(Look::End),
            0b00_0000_0000_0000_0100 => Some(Look::StartLF),
            0b00_0000_0000_0000_1000 => Some(Look::EndLF),
            0b00_0000_0000_0001_0000 => Some(Look::StartCRLF),
            0b00_0000_0000_0010_0000 => Some(Look::EndCRLF),
            0b00_0000_0000_0100_0000 => Some(Look::WordAscii),
            0b00_0000_0000_1000_0000 => Some(Look::WordAsciiNegate),
            0b00_0000_0001_0000_0000 => Some(Look::WordUnicode),
            0b00_0000_0010_0000_0000 => Some(Look::WordUnicodeNegate),
            0b00_0000_0100_0000_0000 => Some(Look::WordStartAscii),
            0b00_0000_1000_0000_0000 => Some(Look::WordEndAscii),
            0b00_0001_0000_0000_0000 => Some(Look::WordStartUnicode),
            0b00_0010_0000_0000_0000 => Some(Look::WordEndUnicode),
            0b00_0100_0000_0000_0000 => Some(Look::WordStartHalfAscii),
            0b00_1000_0000_0000_0000 => Some(Look::WordEndHalfAscii),
            0b01_0000_0000_0000_0000 => Some(Look::WordStartHalfUnicode),
            0b10_0000_0000_0000_0000 => Some(Look::WordEndHalfUnicode),
            _ => None,
        }
    }

    /// Returns a convenient single codepoint representation of this
    /// look-around assertion. Each assertion is guaranteed to be represented
    /// by a distinct character.
    ///
    /// This is useful for succinctly representing a look-around assertion in
    /// human friendly but succinct output intended for a programmer working on
    /// regex internals.
    #[inline]
    pub const fn as_char(self) -> char {
        match self {
            Look::Start => 'A',
            Look::End => 'z',
            Look::StartLF => '^',
            Look::EndLF => '$',
            Look::StartCRLF => 'r',
            Look::EndCRLF => 'R',
            Look::WordAscii => 'b',
            Look::WordAsciiNegate => 'B',
            Look::WordUnicode => '𝛃',
            Look::WordUnicodeNegate => '𝚩',
            Look::WordStartAscii => '<',
            Look::WordEndAscii => '>',
            Look::WordStartUnicode => '〈',
            Look::WordEndUnicode => '〉',
            Look::WordStartHalfAscii => '◁',
            Look::WordEndHalfAscii => '▷',
            Look::WordStartHalfUnicode => '◀',
            Look::WordEndHalfUnicode => '▶',
        }
    }
}

/// LookSet is a memory-efficient set of look-around assertions.
///
/// This is useful for efficiently tracking look-around assertions. For
/// example, a [`thompson::NFA`](crate::nfa::thompson::NFA) provides properties
/// that return `LookSet`s.
#[derive(Clone, Copy, Default, Eq, PartialEq)]
pub struct LookSet {
    /// The underlying representation this set is exposed to make it possible
    /// to store it somewhere efficiently. The representation is that
    /// of a bitset, where each assertion occupies bit `i` where
    /// `i = Look::as_repr()`.
    ///
    /// Note that users of this internal representation must permit the full
    /// range of `u16` values to be represented. For example, even if the
    /// current implementation only makes use of the 10 least significant bits,
    /// it may use more bits in a future semver compatible release.
    pub bits: u32,
}

impl LookSet {
    /// Create an empty set of look-around assertions.
    #[inline]
    pub fn empty() -> LookSet {
        LookSet { bits: 0 }
    }

    /// Create a full set of look-around assertions.
    ///
    /// This set contains all possible look-around assertions.
    #[inline]
    pub fn full() -> LookSet {
        LookSet { bits: !0 }
    }

    /// Create a look-around set containing the look-around assertion given.
    ///
    /// This is a convenience routine for creating an empty set and inserting
    /// one look-around assertions.
    #[inline]
    pub fn singleton(look: Look) -> LookSet {
        LookSet::empty().insert(look)
    }

    /// Returns the total number of look-around assertions in this set.
    #[inline]
    pub fn len(self) -> usize {
        // OK because max value always fits in a u8, which in turn always
        // fits in a usize, regardless of target.
        usize::try_from(self.bits.count_ones()).unwrap()
    }

    /// Returns true if and only if this set is empty.
    #[inline]
    pub fn is_empty(self) -> bool {
        self.len() == 0
    }

    /// Returns true if and only if the given look-around assertion is in this
    /// set.
    #[inline]
    pub fn contains(self, look: Look) -> bool {
        self.bits & look.as_repr() != 0
    }

    /// Returns true if and only if this set contains any anchor assertions.
    /// This includes both "start/end of haystack" and "start/end of line."
    #[inline]
    pub fn contains_anchor(&self) -> bool {
        self.contains_anchor_haystack() || self.contains_anchor_line()
    }

    /// Returns true if and only if this set contains any "start/end of
    /// haystack" anchors. This doesn't include "start/end of line" anchors.
    #[inline]
    pub fn contains_anchor_haystack(&self) -> bool {
        self.contains(Look::Start) || self.contains(Look::End)
    }

    /// Returns true if and only if this set contains any "start/end of line"
    /// anchors. This doesn't include "start/end of haystack" anchors. This
    /// includes both `\n` line anchors and CRLF (`\r\n`) aware line anchors.
    #[inline]
    pub fn contains_anchor_line(&self) -> bool {
        self.contains(Look::StartLF)
            || self.contains(Look::EndLF)
            || self.contains(Look::StartCRLF)
            || self.contains(Look::EndCRLF)
    }

    /// Returns true if and only if this set contains any "start/end of line"
    /// anchors that only treat `\n` as line terminators. This does not include
    /// haystack anchors or CRLF aware line anchors.
    #[inline]
    pub fn contains_anchor_lf(&self) -> bool {
        self.contains(Look::StartLF) || self.contains(Look::EndLF)
    }

    /// Returns true if and only if this set contains any "start/end of line"
    /// anchors that are CRLF-aware. This doesn't include "start/end of
    /// haystack" or "start/end of line-feed" anchors.
    #[inline]
    pub fn contains_anchor_crlf(&self) -> bool {
        self.contains(Look::StartCRLF) || self.contains(Look::EndCRLF)
    }

    /// Returns true if and only if this set contains any word boundary or
    /// negated word boundary assertions. This include both Unicode and ASCII
    /// word boundaries.
    #[inline]
    pub fn contains_word(self) -> bool {
        self.contains_word_unicode() || self.contains_word_ascii()
    }

    /// Returns true if and only if this set contains any Unicode word boundary
    /// or negated Unicode word boundary assertions.
    #[inline]
    pub fn contains_word_unicode(self) -> bool {
        self.contains(Look::WordUnicode)
            || self.contains(Look::WordUnicodeNegate)
            || self.contains(Look::WordStartUnicode)
            || self.contains(Look::WordEndUnicode)
            || self.contains(Look::WordStartHalfUnicode)
            || self.contains(Look::WordEndHalfUnicode)
    }

    /// Returns true if and only if this set contains any ASCII word boundary
    /// or negated ASCII word boundary assertions.
    #[inline]
    pub fn contains_word_ascii(self) -> bool {
        self.contains(Look::WordAscii)
            || self.contains(Look::WordAsciiNegate)
            || self.contains(Look::WordStartAscii)
            || self.contains(Look::WordEndAscii)
            || self.contains(Look::WordStartHalfAscii)
            || self.contains(Look::WordEndHalfAscii)
    }

    /// Returns an iterator over all of the look-around assertions in this set.
    #[inline]
    pub fn iter(self) -> LookSetIter {
        LookSetIter { set: self }
    }

    /// Return a new set that is equivalent to the original, but with the given
    /// assertion added to it. If the assertion is already in the set, then the
    /// returned set is equivalent to the original.
    #[inline]
    pub fn insert(self, look: Look) -> LookSet {
        LookSet { bits: self.bits | look.as_repr() }
    }

    /// Updates this set in place with the result of inserting the given
    /// assertion into this set.
    #[inline]
    pub fn set_insert(&mut self, look: Look) {
        *self = self.insert(look);
    }

    /// Return a new set that is equivalent to the original, but with the given
    /// assertion removed from it. If the assertion is not in the set, then the
    /// returned set is equivalent to the original.
    #[inline]
    pub fn remove(self, look: Look) -> LookSet {
        LookSet { bits: self.bits & !look.as_repr() }
    }

    /// Updates this set in place with the result of removing the given
    /// assertion from this set.
    #[inline]
    pub fn set_remove(&mut self, look: Look) {
        *self = self.remove(look);
    }

    /// Returns a new set that is the result of subtracting the given set from
    /// this set.
    #[inline]
    pub fn subtract(self, other: LookSet) -> LookSet {
        LookSet { bits: self.bits & !other.bits }
    }

    /// Updates this set in place with the result of subtracting the given set
    /// from this set.
    #[inline]
    pub fn set_subtract(&mut self, other: LookSet) {
        *self = self.subtract(other);
    }

    /// Returns a new set that is the union of this and the one given.
    #[inline]
    pub fn union(self, other: LookSet) -> LookSet {
        LookSet { bits: self.bits | other.bits }
    }

    /// Updates this set in place with the result of unioning it with the one
    /// given.
    #[inline]
    pub fn set_union(&mut self, other: LookSet) {
        *self = self.union(other);
    }

    /// Returns a new set that is the intersection of this and the one given.
    #[inline]
    pub fn intersect(self, other: LookSet) -> LookSet {
        LookSet { bits: self.bits & other.bits }
    }

    /// Updates this set in place with the result of intersecting it with the
    /// one given.
    #[inline]
    pub fn set_intersect(&mut self, other: LookSet) {
        *self = self.intersect(other);
    }

    /// Return a `LookSet` from the slice given as a native endian 32-bit
    /// integer.
    ///
    /// # Panics
    ///
    /// This panics if `slice.len() < 4`.
    #[inline]
    pub fn read_repr(slice: &[u8]) -> LookSet {
        let bits = u32::from_ne_bytes(slice[..4].try_into().unwrap());
        LookSet { bits }
    }

    /// Write a `LookSet` as a native endian 32-bit integer to the beginning
    /// of the slice given.
    ///
    /// # Panics
    ///
    /// This panics if `slice.len() < 4`.
    #[inline]
    pub fn write_repr(self, slice: &mut [u8]) {
        let raw = self.bits.to_ne_bytes();
        slice[0] = raw[0];
        slice[1] = raw[1];
        slice[2] = raw[2];
        slice[3] = raw[3];
    }

    /// Checks that all assertions in this set can be matched.
    ///
    /// Some assertions, such as Unicode word boundaries, require optional (but
    /// enabled by default) tables that may not be available. If there are
    /// assertions in this set that require tables that are not available, then
    /// this will return an error.
    ///
    /// Specifically, this returns an error when the the
    /// `unicode-word-boundary` feature is _not_ enabled _and_ this set
    /// contains a Unicode word boundary assertion.
    ///
    /// It can be useful to use this on the result of
    /// [`NFA::look_set_any`](crate::nfa::thompson::NFA::look_set_any)
    /// when building a matcher engine to ensure methods like
    /// [`LookMatcher::matches_set`] do not panic at search time.
    pub fn available(self) -> Result<(), UnicodeWordBoundaryError> {
        if self.contains_word_unicode() {
            UnicodeWordBoundaryError::check()?;
        }
        Ok(())
    }
}

impl core::fmt::Debug for LookSet {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        if self.is_empty() {
            return write!(f, "∅");
        }
        for look in self.iter() {
            write!(f, "{}", look.as_char())?;
        }
        Ok(())
    }
}

/// An iterator over all look-around assertions in a [`LookSet`].
///
/// This iterator is created by [`LookSet::iter`].
#[derive(Clone, Debug)]
pub struct LookSetIter {
    set: LookSet,
}

impl Iterator for LookSetIter {
    type Item = Look;

    #[inline]
    fn next(&mut self) -> Option<Look> {
        if self.set.is_empty() {
            return None;
        }
        // We'll never have more than u8::MAX distinct look-around assertions,
        // so 'bit' will always fit into a u16.
        let bit = u16::try_from(self.set.bits.trailing_zeros()).unwrap();
        let look = Look::from_repr(1 << bit)?;
        self.set = self.set.remove(look);
        Some(look)
    }
}

/// A matcher for look-around assertions.
///
/// This matcher permits configuring aspects of how look-around assertions are
/// matched.
///
/// # Example
///
/// A `LookMatcher` can change the line terminator used for matching multi-line
/// anchors such as `(?m:^)` and `(?m:$)`.
///
/// ```
/// use regex_automata::{
///     nfa::thompson::{self, pikevm::PikeVM},
///     util::look::LookMatcher,
///     Match, Input,
/// };
///
/// let mut lookm = LookMatcher::new();
/// lookm.set_line_terminator(b'\x00');
///
/// let re = PikeVM::builder()
///     .thompson(thompson::Config::new().look_matcher(lookm))
///     .build(r"(?m)^[a-z]+$")?;
/// let mut cache = re.create_cache();
///
/// // Multi-line assertions now use NUL as a terminator.
/// assert_eq!(
///     Some(Match::must(0, 1..4)),
///     re.find(&mut cache, b"\x00abc\x00"),
/// );
/// // ... and \n is no longer recognized as a terminator.
/// assert_eq!(
///     None,
///     re.find(&mut cache, b"\nabc\n"),
/// );
///
/// # Ok::<(), Box<dyn std::error::Error>>(())
/// ```
#[derive(Clone, Debug)]
pub struct LookMatcher {
    lineterm: DebugByte,
}

impl LookMatcher {
    /// Creates a new default matcher for look-around assertions.
    pub fn new() -> LookMatcher {
        LookMatcher { lineterm: DebugByte(b'\n') }
    }

    /// Sets the line terminator for use with `(?m:^)` and `(?m:$)`.
    ///
    /// Namely, instead of `^` matching after `\n` and `$` matching immediately
    /// before a `\n`, this will cause it to match after and before the byte
    /// given.
    ///
    /// It can occasionally be useful to use this to configure the line
    /// terminator to the NUL byte when searching binary data.
    ///
    /// Note that this does not apply to CRLF-aware line anchors such as
    /// `(?Rm:^)` and `(?Rm:$)`. CRLF-aware line anchors are hard-coded to
    /// use `\r` and `\n`.
    pub fn set_line_terminator(&mut self, byte: u8) -> &mut LookMatcher {
        self.lineterm.0 = byte;
        self
    }

    /// Returns the line terminator that was configured for this matcher.
    ///
    /// If no line terminator was configured, then this returns `\n`.
    ///
    /// Note that the line terminator should only be used for matching `(?m:^)`
    /// and `(?m:$)` assertions. It specifically should _not_ be used for
    /// matching the CRLF aware assertions `(?Rm:^)` and `(?Rm:$)`.
    pub fn get_line_terminator(&self) -> u8 {
        self.lineterm.0
    }

    /// Returns true when the position `at` in `haystack` satisfies the given
    /// look-around assertion.
    ///
    /// # Panics
    ///
    /// This panics when testing any Unicode word boundary assertion in this
    /// set and when the Unicode word data is not available. Specifically, this
    /// only occurs when the `unicode-word-boundary` feature is not enabled.
    ///
    /// Since it's generally expected that this routine is called inside of
    /// a matching engine, callers should check the error condition when
    /// building the matching engine. If there is a Unicode word boundary
    /// in the matcher and the data isn't available, then the matcher should
    /// fail to build.
    ///
    /// Callers can check the error condition with [`LookSet::available`].
    ///
    /// This also may panic when `at > haystack.len()`. Note that `at ==
    /// haystack.len()` is legal and guaranteed not to panic.
    #[inline]
    pub fn matches(&self, look: Look, haystack: &[u8], at: usize) -> bool {
        self.matches_inline(look, haystack, at)
    }

    /// Like `matches`, but forcefully inlined.
    ///
    /// # Panics
    ///
    /// This panics when testing any Unicode word boundary assertion in this
    /// set and when the Unicode word data is not available. Specifically, this
    /// only occurs when the `unicode-word-boundary` feature is not enabled.
    ///
    /// Since it's generally expected that this routine is called inside of
    /// a matching engine, callers should check the error condition when
    /// building the matching engine. If there is a Unicode word boundary
    /// in the matcher and the data isn't available, then the matcher should
    /// fail to build.
    ///
    /// Callers can check the error condition with [`LookSet::available`].
    ///
    /// This also may panic when `at > haystack.len()`. Note that `at ==
    /// haystack.len()` is legal and guaranteed not to panic.
    #[cfg_attr(feature = "perf-inline", inline(always))]
    pub(crate) fn matches_inline(
        &self,
        look: Look,
        haystack: &[u8],
        at: usize,
    ) -> bool {
        match look {
            Look::Start => self.is_start(haystack, at),
            Look::End => self.is_end(haystack, at),
            Look::StartLF => self.is_start_lf(haystack, at),
            Look::EndLF => self.is_end_lf(haystack, at),
            Look::StartCRLF => self.is_start_crlf(haystack, at),
            Look::EndCRLF => self.is_end_crlf(haystack, at),
            Look::WordAscii => self.is_word_ascii(haystack, at),
            Look::WordAsciiNegate => self.is_word_ascii_negate(haystack, at),
            Look::WordUnicode => self.is_word_unicode(haystack, at).unwrap(),
            Look::WordUnicodeNegate => {
                self.is_word_unicode_negate(haystack, at).unwrap()
            }
            Look::WordStartAscii => self.is_word_start_ascii(haystack, at),
            Look::WordEndAscii => self.is_word_end_ascii(haystack, at),
            Look::WordStartUnicode => {
                self.is_word_start_unicode(haystack, at).unwrap()
            }
            Look::WordEndUnicode => {
                self.is_word_end_unicode(haystack, at).unwrap()
            }
            Look::WordStartHalfAscii => {
                self.is_word_start_half_ascii(haystack, at)
            }
            Look::WordEndHalfAscii => {
                self.is_word_end_half_ascii(haystack, at)
            }
            Look::WordStartHalfUnicode => {
                self.is_word_start_half_unicode(haystack, at).unwrap()
            }
            Look::WordEndHalfUnicode => {
                self.is_word_end_half_unicode(haystack, at).unwrap()
            }
        }
    }

    /// Returns true when _all_ of the assertions in the given set match at the
    /// given position in the haystack.
    ///
    /// # Panics
    ///
    /// This panics when testing any Unicode word boundary assertion in this
    /// set and when the Unicode word data is not available. Specifically, this
    /// only occurs when the `unicode-word-boundary` feature is not enabled.
    ///
    /// Since it's generally expected that this routine is called inside of
    /// a matching engine, callers should check the error condition when
    /// building the matching engine. If there is a Unicode word boundary
    /// in the matcher and the data isn't available, then the matcher should
    /// fail to build.
    ///
    /// Callers can check the error condition with [`LookSet::available`].
    ///
    /// This also may panic when `at > haystack.len()`. Note that `at ==
    /// haystack.len()` is legal and guaranteed not to panic.
    #[inline]
    pub fn matches_set(
        &self,
        set: LookSet,
        haystack: &[u8],
        at: usize,
    ) -> bool {
        self.matches_set_inline(set, haystack, at)
    }

    /// Like `LookSet::matches`, but forcefully inlined for perf.
    #[cfg_attr(feature = "perf-inline", inline(always))]
    pub(crate) fn matches_set_inline(
        &self,
        set: LookSet,
        haystack: &[u8],
        at: usize,
    ) -> bool {
        // This used to luse LookSet::iter with Look::matches on each element,
        // but that proved to be quite diastrous for perf. The manual "if
        // the set has this assertion, check it" turns out to be quite a bit
        // faster.
        if set.contains(Look::Start) {
            if !self.is_start(haystack, at) {
                return false;
            }
        }
        if set.contains(Look::End) {
            if !self.is_end(haystack, at) {
                return false;
            }
        }
        if set.contains(Look::StartLF) {
            if !self.is_start_lf(haystack, at) {
                return false;
            }
        }
        if set.contains(Look::EndLF) {
            if !self.is_end_lf(haystack, at) {
                return false;
            }
        }
        if set.contains(Look::StartCRLF) {
            if !self.is_start_crlf(haystack, at) {
                return false;
            }
        }
        if set.contains(Look::EndCRLF) {
            if !self.is_end_crlf(haystack, at) {
                return false;
            }
        }
        if set.contains(Look::WordAscii) {
            if !self.is_word_ascii(haystack, at) {
                return false;
            }
        }
        if set.contains(Look::WordAsciiNegate) {
            if !self.is_word_ascii_negate(haystack, at) {
                return false;
            }
        }
        if set.contains(Look::WordUnicode) {
            if !self.is_word_unicode(haystack, at).unwrap() {
                return false;
            }
        }
        if set.contains(Look::WordUnicodeNegate) {
            if !self.is_word_unicode_negate(haystack, at).unwrap() {
                return false;
            }
        }
        if set.contains(Look::WordStartAscii) {
            if !self.is_word_start_ascii(haystack, at) {
                return false;
            }
        }
        if set.contains(Look::WordEndAscii) {
            if !self.is_word_end_ascii(haystack, at) {
                return false;
            }
        }
        if set.contains(Look::WordStartUnicode) {
            if !self.is_word_start_unicode(haystack, at).unwrap() {
                return false;
            }
        }
        if set.contains(Look::WordEndUnicode) {
            if !self.is_word_end_unicode(haystack, at).unwrap() {
                return false;
            }
        }
        if set.contains(Look::WordStartHalfAscii) {
            if !self.is_word_start_half_ascii(haystack, at) {
                return false;
            }
        }
        if set.contains(Look::WordEndHalfAscii) {
            if !self.is_word_end_half_ascii(haystack, at) {
                return false;
            }
        }
        if set.contains(Look::WordStartHalfUnicode) {
            if !self.is_word_start_half_unicode(haystack, at).unwrap() {
                return false;
            }
        }
        if set.contains(Look::WordEndHalfUnicode) {
            if !self.is_word_end_half_unicode(haystack, at).unwrap() {
                return false;
            }
        }
        true
    }

    /// Split up the given byte classes into equivalence classes in a way that
    /// is consistent with this look-around assertion.
    #[cfg(feature = "alloc")]
    pub(crate) fn add_to_byteset(
        &self,
        look: Look,
        set: &mut crate::util::alphabet::ByteClassSet,
    ) {
        match look {
            Look::Start | Look::End => {}
            Look::StartLF | Look::EndLF => {
                set.set_range(self.lineterm.0, self.lineterm.0);
            }
            Look::StartCRLF | Look::EndCRLF => {
                set.set_range(b'\r', b'\r');
                set.set_range(b'\n', b'\n');
            }
            Look::WordAscii
            | Look::WordAsciiNegate
            | Look::WordUnicode
            | Look::WordUnicodeNegate
            | Look::WordStartAscii
            | Look::WordEndAscii
            | Look::WordStartUnicode
            | Look::WordEndUnicode
            | Look::WordStartHalfAscii
            | Look::WordEndHalfAscii
            | Look::WordStartHalfUnicode
            | Look::WordEndHalfUnicode => {
                // We need to mark all ranges of bytes whose pairs result in
                // evaluating \b differently. This isn't technically correct
                // for Unicode word boundaries, but DFAs can't handle those
                // anyway, and thus, the byte classes don't need to either
                // since they are themselves only used in DFAs.
                //
                // FIXME: It seems like the calls to 'set_range' here are
                // completely invariant, which means we could just hard-code
                // them here without needing to write a loop. And we only need
                // to do this dance at most once per regex.
                //
                // FIXME: Is this correct for \B?
                let iswb = utf8::is_word_byte;
                // This unwrap is OK because we guard every use of 'asu8' with
                // a check that the input is <= 255.
                let asu8 = |b: u16| u8::try_from(b).unwrap();
                let mut b1: u16 = 0;
                let mut b2: u16;
                while b1 <= 255 {
                    b2 = b1 + 1;
                    while b2 <= 255 && iswb(asu8(b1)) == iswb(asu8(b2)) {
                        b2 += 1;
                    }
                    // The guards above guarantee that b2 can never get any
                    // bigger.
                    assert!(b2 <= 256);
                    // Subtracting 1 from b2 is always OK because it is always
                    // at least 1 greater than b1, and the assert above
                    // guarantees that the asu8 conversion will succeed.
                    set.set_range(asu8(b1), asu8(b2.checked_sub(1).unwrap()));
                    b1 = b2;
                }
            }
        }
    }

    /// Returns true when [`Look::Start`] is satisfied `at` the given position
    /// in `haystack`.
    ///
    /// # Panics
    ///
    /// This may panic when `at > haystack.len()`. Note that `at ==
    /// haystack.len()` is legal and guaranteed not to panic.
    #[inline]
    pub fn is_start(&self, _haystack: &[u8], at: usize) -> bool {
        at == 0
    }

    /// Returns true when [`Look::End`] is satisfied `at` the given position in
    /// `haystack`.
    ///
    /// # Panics
    ///
    /// This may panic when `at > haystack.len()`. Note that `at ==
    /// haystack.len()` is legal and guaranteed not to panic.
    #[inline]
    pub fn is_end(&self, haystack: &[u8], at: usize) -> bool {
        at == haystack.len()
    }

    /// Returns true when [`Look::StartLF`] is satisfied `at` the given
    /// position in `haystack`.
    ///
    /// # Panics
    ///
    /// This may panic when `at > haystack.len()`. Note that `at ==
    /// haystack.len()` is legal and guaranteed not to panic.
    #[inline]
    pub fn is_start_lf(&self, haystack: &[u8], at: usize) -> bool {
        self.is_start(haystack, at) || haystack[at - 1] == self.lineterm.0
    }

    /// Returns true when [`Look::EndLF`] is satisfied `at` the given position
    /// in `haystack`.
    ///
    /// # Panics
    ///
    /// This may panic when `at > haystack.len()`. Note that `at ==
    /// haystack.len()` is legal and guaranteed not to panic.
    #[inline]
    pub fn is_end_lf(&self, haystack: &[u8], at: usize) -> bool {
        self.is_end(haystack, at) || haystack[at] == self.lineterm.0
    }

    /// Returns true when [`Look::StartCRLF`] is satisfied `at` the given
    /// position in `haystack`.
    ///
    /// # Panics
    ///
    /// This may panic when `at > haystack.len()`. Note that `at ==
    /// haystack.len()` is legal and guaranteed not to panic.
    #[inline]
    pub fn is_start_crlf(&self, haystack: &[u8], at: usize) -> bool {
        self.is_start(haystack, at)
            || haystack[at - 1] == b'\n'
            || (haystack[at - 1] == b'\r'
                && (at >= haystack.len() || haystack[at] != b'\n'))
    }

    /// Returns true when [`Look::EndCRLF`] is satisfied `at` the given
    /// position in `haystack`.
    ///
    /// # Panics
    ///
    /// This may panic when `at > haystack.len()`. Note that `at ==
    /// haystack.len()` is legal and guaranteed not to panic.
    #[inline]
    pub fn is_end_crlf(&self, haystack: &[u8], at: usize) -> bool {
        self.is_end(haystack, at)
            || haystack[at] == b'\r'
            || (haystack[at] == b'\n'
                && (at == 0 || haystack[at - 1] != b'\r'))
    }

    /// Returns true when [`Look::WordAscii`] is satisfied `at` the given
    /// position in `haystack`.
    ///
    /// # Panics
    ///
    /// This may panic when `at > haystack.len()`. Note that `at ==
    /// haystack.len()` is legal and guaranteed not to panic.
    #[inline]
    pub fn is_word_ascii(&self, haystack: &[u8], at: usize) -> bool {
        let word_before = at > 0 && utf8::is_word_byte(haystack[at - 1]);
        let word_after =
            at < haystack.len() && utf8::is_word_byte(haystack[at]);
        word_before != word_after
    }

    /// Returns true when [`Look::WordAsciiNegate`] is satisfied `at` the given
    /// position in `haystack`.
    ///
    /// # Panics
    ///
    /// This may panic when `at > haystack.len()`. Note that `at ==
    /// haystack.len()` is legal and guaranteed not to panic.
    #[inline]
    pub fn is_word_ascii_negate(&self, haystack: &[u8], at: usize) -> bool {
        !self.is_word_ascii(haystack, at)
    }

    /// Returns true when [`Look::WordUnicode`] is satisfied `at` the given
    /// position in `haystack`.
    ///
    /// # Panics
    ///
    /// This may panic when `at > haystack.len()`. Note that `at ==
    /// haystack.len()` is legal and guaranteed not to panic.
    ///
    /// # Errors
    ///
    /// This returns an error when Unicode word boundary tables
    /// are not available. Specifically, this only occurs when the
    /// `unicode-word-boundary` feature is not enabled.
    #[inline]
    pub fn is_word_unicode(
        &self,
        haystack: &[u8],
        at: usize,
    ) -> Result<bool, UnicodeWordBoundaryError> {
        let word_before = is_word_char::rev(haystack, at)?;
        let word_after = is_word_char::fwd(haystack, at)?;
        Ok(word_before != word_after)
    }

    /// Returns true when [`Look::WordUnicodeNegate`] is satisfied `at` the
    /// given position in `haystack`.
    ///
    /// # Panics
    ///
    /// This may panic when `at > haystack.len()`. Note that `at ==
    /// haystack.len()` is legal and guaranteed not to panic.
    ///
    /// # Errors
    ///
    /// This returns an error when Unicode word boundary tables
    /// are not available. Specifically, this only occurs when the
    /// `unicode-word-boundary` feature is not enabled.
    #[inline]
    pub fn is_word_unicode_negate(
        &self,
        haystack: &[u8],
        at: usize,
    ) -> Result<bool, UnicodeWordBoundaryError> {
        // This is pretty subtle. Why do we need to do UTF-8 decoding here?
        // Well... at time of writing, the is_word_char_{fwd,rev} routines will
        // only return true if there is a valid UTF-8 encoding of a "word"
        // codepoint, and false in every other case (including invalid UTF-8).
        // This means that in regions of invalid UTF-8 (which might be a
        // subset of valid UTF-8!), it would result in \B matching. While this
        // would be questionable in the context of truly invalid UTF-8, it is
        // *certainly* wrong to report match boundaries that split the encoding
        // of a codepoint. So to work around this, we ensure that we can decode
        // a codepoint on either side of `at`. If either direction fails, then
        // we don't permit \B to match at all.
        //
        // Now, this isn't exactly optimal from a perf perspective. We could
        // try and detect this in is_word_char::{fwd,rev}, but it's not clear
        // if it's worth it. \B is, after all, rarely used. Even worse,
        // is_word_char::{fwd,rev} could do its own UTF-8 decoding, and so this
        // will wind up doing UTF-8 decoding twice. Owch. We could fix this
        // with more code complexity, but it just doesn't feel worth it for \B.
        //
        // And in particular, we do *not* have to do this with \b, because \b
        // *requires* that at least one side of `at` be a "word" codepoint,
        // which in turn implies one side of `at` must be valid UTF-8. This in
        // turn implies that \b can never split a valid UTF-8 encoding of a
        // codepoint. In the case where one side of `at` is truly invalid UTF-8
        // and the other side IS a word codepoint, then we want \b to match
        // since it represents a valid UTF-8 boundary. It also makes sense. For
        // example, you'd want \b\w+\b to match 'abc' in '\xFFabc\xFF'.
        //
        // Note also that this is not just '!is_word_unicode(..)' like it is
        // for the ASCII case. For example, neither \b nor \B is satisfied
        // within invalid UTF-8 sequences.
        let word_before = at > 0
            && match utf8::decode_last(&haystack[..at]) {
                None | Some(Err(_)) => return Ok(false),
                Some(Ok(_)) => is_word_char::rev(haystack, at)?,
            };
        let word_after = at < haystack.len()
            && match utf8::decode(&haystack[at..]) {
                None | Some(Err(_)) => return Ok(false),
                Some(Ok(_)) => is_word_char::fwd(haystack, at)?,
            };
        Ok(word_before == word_after)
    }

    /// Returns true when [`Look::WordStartAscii`] is satisfied `at` the given
    /// position in `haystack`.
    ///
    /// # Panics
    ///
    /// This may panic when `at > haystack.len()`. Note that `at ==
    /// haystack.len()` is legal and guaranteed not to panic.
    #[inline]
    pub fn is_word_start_ascii(&self, haystack: &[u8], at: usize) -> bool {
        let word_before = at > 0 && utf8::is_word_byte(haystack[at - 1]);
        let word_after =
            at < haystack.len() && utf8::is_word_byte(haystack[at]);
        !word_before && word_after
    }

    /// Returns true when [`Look::WordEndAscii`] is satisfied `at` the given
    /// position in `haystack`.
    ///
    /// # Panics
    ///
    /// This may panic when `at > haystack.len()`. Note that `at ==
    /// haystack.len()` is legal and guaranteed not to panic.
    #[inline]
    pub fn is_word_end_ascii(&self, haystack: &[u8], at: usize) -> bool {
        let word_before = at > 0 && utf8::is_word_byte(haystack[at - 1]);
        let word_after =
            at < haystack.len() && utf8::is_word_byte(haystack[at]);
        word_before && !word_after
    }

    /// Returns true when [`Look::WordStartUnicode`] is satisfied `at` the
    /// given position in `haystack`.
    ///
    /// # Panics
    ///
    /// This may panic when `at > haystack.len()`. Note that `at ==
    /// haystack.len()` is legal and guaranteed not to panic.
    ///
    /// # Errors
    ///
    /// This returns an error when Unicode word boundary tables
    /// are not available. Specifically, this only occurs when the
    /// `unicode-word-boundary` feature is not enabled.
    #[inline]
    pub fn is_word_start_unicode(
        &self,
        haystack: &[u8],
        at: usize,
    ) -> Result<bool, UnicodeWordBoundaryError> {
        let word_before = is_word_char::rev(haystack, at)?;
        let word_after = is_word_char::fwd(haystack, at)?;
        Ok(!word_before && word_after)
    }

    /// Returns true when [`Look::WordEndUnicode`] is satisfied `at` the
    /// given position in `haystack`.
    ///
    /// # Panics
    ///
    /// This may panic when `at > haystack.len()`. Note that `at ==
    /// haystack.len()` is legal and guaranteed not to panic.
    ///
    /// # Errors
    ///
    /// This returns an error when Unicode word boundary tables
    /// are not available. Specifically, this only occurs when the
    /// `unicode-word-boundary` feature is not enabled.
    #[inline]
    pub fn is_word_end_unicode(
        &self,
        haystack: &[u8],
        at: usize,
    ) -> Result<bool, UnicodeWordBoundaryError> {
        let word_before = is_word_char::rev(haystack, at)?;
        let word_after = is_word_char::fwd(haystack, at)?;
        Ok(word_before && !word_after)
    }

    /// Returns true when [`Look::WordStartHalfAscii`] is satisfied `at` the
    /// given position in `haystack`.
    ///
    /// # Panics
    ///
    /// This may panic when `at > haystack.len()`. Note that `at ==
    /// haystack.len()` is legal and guaranteed not to panic.
    #[inline]
    pub fn is_word_start_half_ascii(
        &self,
        haystack: &[u8],
        at: usize,
    ) -> bool {
        let word_before = at > 0 && utf8::is_word_byte(haystack[at - 1]);
        !word_before
    }

    /// Returns true when [`Look::WordEndHalfAscii`] is satisfied `at` the
    /// given position in `haystack`.
    ///
    /// # Panics
    ///
    /// This may panic when `at > haystack.len()`. Note that `at ==
    /// haystack.len()` is legal and guaranteed not to panic.
    #[inline]
    pub fn is_word_end_half_ascii(&self, haystack: &[u8], at: usize) -> bool {
        let word_after =
            at < haystack.len() && utf8::is_word_byte(haystack[at]);
        !word_after
    }

    /// Returns true when [`Look::WordStartHalfUnicode`] is satisfied `at` the
    /// given position in `haystack`.
    ///
    /// # Panics
    ///
    /// This may panic when `at > haystack.len()`. Note that `at ==
    /// haystack.len()` is legal and guaranteed not to panic.
    ///
    /// # Errors
    ///
    /// This returns an error when Unicode word boundary tables
    /// are not available. Specifically, this only occurs when the
    /// `unicode-word-boundary` feature is not enabled.
    #[inline]
    pub fn is_word_start_half_unicode(
        &self,
        haystack: &[u8],
        at: usize,
    ) -> Result<bool, UnicodeWordBoundaryError> {
        // See `is_word_unicode_negate` for why we need to do this. We don't
        // need to do it for `is_word_start_unicode` because that guarantees
        // that the position matched falls on a valid UTF-8 boundary given
        // that the right side must be in \w.
        let word_before = at > 0
            && match utf8::decode_last(&haystack[..at]) {
                None | Some(Err(_)) => return Ok(false),
                Some(Ok(_)) => is_word_char::rev(haystack, at)?,
            };
        Ok(!word_before)
    }

    /// Returns true when [`Look::WordEndHalfUnicode`] is satisfied `at` the
    /// given position in `haystack`.
    ///
    /// # Panics
    ///
    /// This may panic when `at > haystack.len()`. Note that `at ==
    /// haystack.len()` is legal and guaranteed not to panic.
    ///
    /// # Errors
    ///
    /// This returns an error when Unicode word boundary tables
    /// are not available. Specifically, this only occurs when the
    /// `unicode-word-boundary` feature is not enabled.
    #[inline]
    pub fn is_word_end_half_unicode(
        &self,
        haystack: &[u8],
        at: usize,
    ) -> Result<bool, UnicodeWordBoundaryError> {
        // See `is_word_unicode_negate` for why we need to do this. We don't
        // need to do it for `is_word_end_unicode` because that guarantees
        // that the position matched falls on a valid UTF-8 boundary given
        // that the left side must be in \w.
        let word_after = at < haystack.len()
            && match utf8::decode(&haystack[at..]) {
                None | Some(Err(_)) => return Ok(false),
                Some(Ok(_)) => is_word_char::fwd(haystack, at)?,
            };
        Ok(!word_after)
    }
}

impl Default for LookMatcher {
    fn default() -> LookMatcher {
        LookMatcher::new()
    }
}

/// An error that occurs when the Unicode-aware `\w` class is unavailable.
///
/// This error can occur when the data tables necessary for the Unicode aware
/// Perl character class `\w` are unavailable. The `\w` class is used to
/// determine whether a codepoint is considered a word character or not when
/// determining whether a Unicode aware `\b` (or `\B`) matches at a particular
/// position.
///
/// This error can only occur when the `unicode-word-boundary` feature is
/// disabled.
#[derive(Clone, Debug)]
pub struct UnicodeWordBoundaryError(());

impl UnicodeWordBoundaryError {
    #[cfg(not(feature = "unicode-word-boundary"))]
    pub(crate) fn new() -> UnicodeWordBoundaryError {
        UnicodeWordBoundaryError(())
    }

    /// Returns an error if and only if Unicode word boundary data is
    /// unavailable.
    pub fn check() -> Result<(), UnicodeWordBoundaryError> {
        is_word_char::check()
    }
}

#[cfg(feature = "std")]
impl std::error::Error for UnicodeWordBoundaryError {}

impl core::fmt::Display for UnicodeWordBoundaryError {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        write!(
            f,
            "Unicode-aware \\b and \\B are unavailable because the \
             requisite data tables are missing, please enable the \
             unicode-word-boundary feature"
        )
    }
}

// Below are FOUR different ways for checking whether whether a "word"
// codepoint exists at a particular position in the haystack. The four
// different approaches are, in order of preference:
//
// 1. Parse '\w', convert to an NFA, convert to a fully compiled DFA on the
// first call, and then use that DFA for all subsequent calls.
// 2. Do UTF-8 decoding and use regex_syntax::is_word_character if available.
// 3. Do UTF-8 decoding and use our own 'perl_word' table.
// 4. Return an error.
//
// The reason for all of these approaches is a combination of perf and
// permitting one to build regex-automata without the Unicode data necessary
// for handling Unicode-aware word boundaries. (In which case, '(?-u:\b)' would
// still work.)
//
// The DFA approach is the fastest, but it requires the regex parser, the
// NFA compiler, the DFA builder and the DFA search runtime. That's a lot to
// bring in, but if it's available, it's (probably) the best we can do.
//
// Approaches (2) and (3) are effectively equivalent, but (2) reuses the
// data in regex-syntax and avoids duplicating it in regex-automata.
//
// Finally, (4) unconditionally returns an error since the requisite data isn't
// available anywhere.
//
// There are actually more approaches possible that we didn't implement. For
// example, if the DFA builder is available but the syntax parser is not, we
// could technically hand construct our own NFA from the 'perl_word' data
// table. But to avoid some pretty hairy code duplication, we would in turn
// need to pull the UTF-8 compiler out of the NFA compiler. Yikes.
//
// A possibly more sensible alternative is to use a lazy DFA when the full
// DFA builder isn't available...
//
// Yet another choice would be to build the full DFA and then embed it into the
// source. Then we'd only need to bring in the DFA search runtime, which is
// considerably smaller than the DFA builder code. The problem here is that the
// Debian people have spooked me[1] into avoiding cyclic dependencies. Namely,
// we'd need to build regex-cli, which depends on regex-automata in order to
// build some part of regex-automata. But to be honest, something like this has
// to be allowed somehow? I just don't know what the right process is.
//
// There are perhaps other choices as well. Why did I stop at these 4? Because
// I wanted to preserve my sanity. I suspect I'll wind up adding the lazy DFA
// approach eventually, as the benefits of the DFA approach are somewhat
// compelling. The 'boundary-words-holmes' benchmark tests this. (Note that
// the commands below no longer work. If necessary, we should re-capitulate
// the benchmark from whole cloth in rebar.)
//
//   $ regex-cli bench measure -f boundary-words-holmes -e pikevm > dfa.csv
//
// Then I changed the code below so that the util/unicode_data/perl_word table
// was used and re-ran the benchmark:
//
//   $ regex-cli bench measure -f boundary-words-holmes -e pikevm > table.csv
//
// And compared them:
//
//   $ regex-cli bench diff dfa.csv table.csv
//   benchmark                             engine                 dfa        table
//   ---------                             ------                 ---        -----
//   internal/count/boundary-words-holmes  regex/automata/pikevm  18.6 MB/s  12.9 MB/s
//
// Which is a nice improvement.
//
// UPDATE: It turns out that it takes approximately 22ms to build the reverse
// DFA for \w. (And about 3ms for the forward DFA.) It's probably not much in
// the grand scheme things, but that is a significant latency cost. So I'm not
// sure that's a good idea. I then tried using a lazy DFA instead, and that
// eliminated the overhead, but since the lazy DFA requires mutable working
// memory, that requires introducing a 'Cache' for every simultaneous call.
//
// I ended up deciding for now to just keep the "UTF-8 decode and check the
// table." The DFA and lazy DFA approaches are still below, but commented out.
//
// [1]: https://github.com/BurntSushi/ucd-generate/issues/11

/*
/// A module that looks for word codepoints using lazy DFAs.
#[cfg(all(
    feature = "unicode-word-boundary",
    feature = "syntax",
    feature = "unicode-perl",
    feature = "hybrid"
))]
mod is_word_char {
    use alloc::vec::Vec;

    use crate::{
        hybrid::dfa::{Cache, DFA},
        nfa::thompson::NFA,
        util::{lazy::Lazy, pool::Pool, primitives::StateID},
        Anchored, Input,
    };

    pub(super) fn check() -> Result<(), super::UnicodeWordBoundaryError> {
        Ok(())
    }

    #[cfg_attr(feature = "perf-inline", inline(always))]
    pub(super) fn fwd(
        haystack: &[u8],
        mut at: usize,
    ) -> Result<bool, super::UnicodeWordBoundaryError> {
        static WORD: Lazy<DFA> = Lazy::new(|| DFA::new(r"\w").unwrap());
        static CACHE: Lazy<Pool<Cache>> =
            Lazy::new(|| Pool::new(|| WORD.create_cache()));
        let dfa = Lazy::get(&WORD);
        let mut cache = Lazy::get(&CACHE).get();
        let mut sid = dfa
            .start_state_forward(
                &mut cache,
                &Input::new("").anchored(Anchored::Yes),
            )
            .unwrap();
        while at < haystack.len() {
            let byte = haystack[at];
            sid = dfa.next_state(&mut cache, sid, byte).unwrap();
            at += 1;
            if sid.is_tagged() {
                if sid.is_match() {
                    return Ok(true);
                } else if sid.is_dead() {
                    return Ok(false);
                }
            }
        }
        Ok(dfa.next_eoi_state(&mut cache, sid).unwrap().is_match())
    }

    #[cfg_attr(feature = "perf-inline", inline(always))]
    pub(super) fn rev(
        haystack: &[u8],
        mut at: usize,
    ) -> Result<bool, super::UnicodeWordBoundaryError> {
        static WORD: Lazy<DFA> = Lazy::new(|| {
            DFA::builder()
                .thompson(NFA::config().reverse(true))
                .build(r"\w")
                .unwrap()
        });
        static CACHE: Lazy<Pool<Cache>> =
            Lazy::new(|| Pool::new(|| WORD.create_cache()));
        let dfa = Lazy::get(&WORD);
        let mut cache = Lazy::get(&CACHE).get();
        let mut sid = dfa
            .start_state_reverse(
                &mut cache,
                &Input::new("").anchored(Anchored::Yes),
            )
            .unwrap();
        while at > 0 {
            at -= 1;
            let byte = haystack[at];
            sid = dfa.next_state(&mut cache, sid, byte).unwrap();
            if sid.is_tagged() {
                if sid.is_match() {
                    return Ok(true);
                } else if sid.is_dead() {
                    return Ok(false);
                }
            }
        }
        Ok(dfa.next_eoi_state(&mut cache, sid).unwrap().is_match())
    }
}
*/

/*
/// A module that looks for word codepoints using fully compiled DFAs.
#[cfg(all(
    feature = "unicode-word-boundary",
    feature = "syntax",
    feature = "unicode-perl",
    feature = "dfa-build"
))]
mod is_word_char {
    use alloc::vec::Vec;

    use crate::{
        dfa::{dense::DFA, Automaton, StartKind},
        nfa::thompson::NFA,
        util::{lazy::Lazy, primitives::StateID},
        Anchored, Input,
    };

    pub(super) fn check() -> Result<(), super::UnicodeWordBoundaryError> {
        Ok(())
    }

    #[cfg_attr(feature = "perf-inline", inline(always))]
    pub(super) fn fwd(
        haystack: &[u8],
        mut at: usize,
    ) -> Result<bool, super::UnicodeWordBoundaryError> {
        static WORD: Lazy<(DFA<Vec<u32>>, StateID)> = Lazy::new(|| {
            let dfa = DFA::builder()
                .configure(DFA::config().start_kind(StartKind::Anchored))
                .build(r"\w")
                .unwrap();
            // OK because our regex has no look-around.
            let start_id = dfa.universal_start_state(Anchored::Yes).unwrap();
            (dfa, start_id)
        });
        let &(ref dfa, mut sid) = Lazy::get(&WORD);
        while at < haystack.len() {
            let byte = haystack[at];
            sid = dfa.next_state(sid, byte);
            at += 1;
            if dfa.is_special_state(sid) {
                if dfa.is_match_state(sid) {
                    return Ok(true);
                } else if dfa.is_dead_state(sid) {
                    return Ok(false);
                }
            }
        }
        Ok(dfa.is_match_state(dfa.next_eoi_state(sid)))
    }

    #[cfg_attr(feature = "perf-inline", inline(always))]
    pub(super) fn rev(
        haystack: &[u8],
        mut at: usize,
    ) -> Result<bool, super::UnicodeWordBoundaryError> {
        static WORD: Lazy<(DFA<Vec<u32>>, StateID)> = Lazy::new(|| {
            let dfa = DFA::builder()
                .configure(DFA::config().start_kind(StartKind::Anchored))
                // From ad hoc measurements, it looks like setting
                // shrink==false is slightly faster than shrink==true. I kind
                // of feel like this indicates that shrinking is probably a
                // failure, although it can help in some cases. Sigh.
                .thompson(NFA::config().reverse(true).shrink(false))
                .build(r"\w")
                .unwrap();
            // OK because our regex has no look-around.
            let start_id = dfa.universal_start_state(Anchored::Yes).unwrap();
            (dfa, start_id)
        });
        let &(ref dfa, mut sid) = Lazy::get(&WORD);
        while at > 0 {
            at -= 1;
            let byte = haystack[at];
            sid = dfa.next_state(sid, byte);
            if dfa.is_special_state(sid) {
                if dfa.is_match_state(sid) {
                    return Ok(true);
                } else if dfa.is_dead_state(sid) {
                    return Ok(false);
                }
            }
        }
        Ok(dfa.is_match_state(dfa.next_eoi_state(sid)))
    }
}
*/

/// A module that looks for word codepoints using regex-syntax's data tables.
#[cfg(all(
    feature = "unicode-word-boundary",
    feature = "syntax",
    feature = "unicode-perl",
))]
mod is_word_char {
    use regex_syntax::try_is_word_character;

    use crate::util::utf8;

    pub(super) fn check() -> Result<(), super::UnicodeWordBoundaryError> {
        Ok(())
    }

    #[cfg_attr(feature = "perf-inline", inline(always))]
    pub(super) fn fwd(
        haystack: &[u8],
        at: usize,
    ) -> Result<bool, super::UnicodeWordBoundaryError> {
        Ok(match utf8::decode(&haystack[at..]) {
            None | Some(Err(_)) => false,
            Some(Ok(ch)) => try_is_word_character(ch).expect(
                "since unicode-word-boundary, syntax and unicode-perl \
                 are all enabled, it is expected that \
                 try_is_word_character succeeds",
            ),
        })
    }

    #[cfg_attr(feature = "perf-inline", inline(always))]
    pub(super) fn rev(
        haystack: &[u8],
        at: usize,
    ) -> Result<bool, super::UnicodeWordBoundaryError> {
        Ok(match utf8::decode_last(&haystack[..at]) {
            None | Some(Err(_)) => false,
            Some(Ok(ch)) => try_is_word_character(ch).expect(
                "since unicode-word-boundary, syntax and unicode-perl \
                 are all enabled, it is expected that \
                 try_is_word_character succeeds",
            ),
        })
    }
}

/// A module that looks for word codepoints using regex-automata's data tables
/// (which are only compiled when regex-syntax's tables aren't available).
///
/// Note that the cfg should match the one in src/util/unicode_data/mod.rs for
/// perl_word.
#[cfg(all(
    feature = "unicode-word-boundary",
    not(all(feature = "syntax", feature = "unicode-perl")),
))]
mod is_word_char {
    use crate::util::utf8;

    pub(super) fn check() -> Result<(), super::UnicodeWordBoundaryError> {
        Ok(())
    }

    #[cfg_attr(feature = "perf-inline", inline(always))]
    pub(super) fn fwd(
        haystack: &[u8],
        at: usize,
    ) -> Result<bool, super::UnicodeWordBoundaryError> {
        Ok(match utf8::decode(&haystack[at..]) {
            None | Some(Err(_)) => false,
            Some(Ok(ch)) => is_word_character(ch),
        })
    }

    #[cfg_attr(feature = "perf-inline", inline(always))]
    pub(super) fn rev(
        haystack: &[u8],
        at: usize,
    ) -> Result<bool, super::UnicodeWordBoundaryError> {
        Ok(match utf8::decode_last(&haystack[..at]) {
            None | Some(Err(_)) => false,
            Some(Ok(ch)) => is_word_character(ch),
        })
    }

    #[cfg_attr(feature = "perf-inline", inline(always))]
    fn is_word_character(c: char) -> bool {
        use crate::util::{unicode_data::perl_word::PERL_WORD, utf8};

        if u8::try_from(c).map_or(false, utf8::is_word_byte) {
            return true;
        }
        PERL_WORD
            .binary_search_by(|&(start, end)| {
                use core::cmp::Ordering;

                if start <= c && c <= end {
                    Ordering::Equal
                } else if start > c {
                    Ordering::Greater
                } else {
                    Ordering::Less
                }
            })
            .is_ok()
    }
}

/// A module that always returns an error if Unicode word boundaries are
/// disabled. When this feature is disabled, then regex-automata will not
/// include its own data tables even if regex-syntax is disabled.
#[cfg(not(feature = "unicode-word-boundary"))]
mod is_word_char {
    pub(super) fn check() -> Result<(), super::UnicodeWordBoundaryError> {
        Err(super::UnicodeWordBoundaryError::new())
    }

    #[cfg_attr(feature = "perf-inline", inline(always))]
    pub(super) fn fwd(
        _bytes: &[u8],
        _at: usize,
    ) -> Result<bool, super::UnicodeWordBoundaryError> {
        Err(super::UnicodeWordBoundaryError::new())
    }

    #[cfg_attr(feature = "perf-inline", inline(always))]
    pub(super) fn rev(
        _bytes: &[u8],
        _at: usize,
    ) -> Result<bool, super::UnicodeWordBoundaryError> {
        Err(super::UnicodeWordBoundaryError::new())
    }
}

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

    macro_rules! testlook {
        ($look:expr, $haystack:expr, $at:expr) => {
            LookMatcher::default().matches($look, $haystack.as_bytes(), $at)
        };
    }

    #[test]
    fn look_matches_start_line() {
        let look = Look::StartLF;

        assert!(testlook!(look, "", 0));
        assert!(testlook!(look, "\n", 0));
        assert!(testlook!(look, "\n", 1));
        assert!(testlook!(look, "a", 0));
        assert!(testlook!(look, "\na", 1));

        assert!(!testlook!(look, "a", 1));
        assert!(!testlook!(look, "a\na", 1));
    }

    #[test]
    fn look_matches_end_line() {
        let look = Look::EndLF;

        assert!(testlook!(look, "", 0));
        assert!(testlook!(look, "\n", 1));
        assert!(testlook!(look, "\na", 0));
        assert!(testlook!(look, "\na", 2));
        assert!(testlook!(look, "a\na", 1));

        assert!(!testlook!(look, "a", 0));
        assert!(!testlook!(look, "\na", 1));
        assert!(!testlook!(look, "a\na", 0));
        assert!(!testlook!(look, "a\na", 2));
    }

    #[test]
    fn look_matches_start_text() {
        let look = Look::Start;

        assert!(testlook!(look, "", 0));
        assert!(testlook!(look, "\n", 0));
        assert!(testlook!(look, "a", 0));

        assert!(!testlook!(look, "\n", 1));
        assert!(!testlook!(look, "\na", 1));
        assert!(!testlook!(look, "a", 1));
        assert!(!testlook!(look, "a\na", 1));
    }

    #[test]
    fn look_matches_end_text() {
        let look = Look::End;

        assert!(testlook!(look, "", 0));
        assert!(testlook!(look, "\n", 1));
        assert!(testlook!(look, "\na", 2));

        assert!(!testlook!(look, "\na", 0));
        assert!(!testlook!(look, "a\na", 1));
        assert!(!testlook!(look, "a", 0));
        assert!(!testlook!(look, "\na", 1));
        assert!(!testlook!(look, "a\na", 0));
        assert!(!testlook!(look, "a\na", 2));
    }

    #[test]
    #[cfg(all(not(miri), feature = "unicode-word-boundary"))]
    fn look_matches_word_unicode() {
        let look = Look::WordUnicode;

        // \xF0\x9D\x9B\x83 = 𝛃 (in \w)
        // \xF0\x90\x86\x80 = 𐆀 (not in \w)

        // Simple ASCII word boundaries.
        assert!(testlook!(look, "a", 0));
        assert!(testlook!(look, "a", 1));
        assert!(testlook!(look, "a ", 1));
        assert!(testlook!(look, " a ", 1));
        assert!(testlook!(look, " a ", 2));

        // Unicode word boundaries with a non-ASCII codepoint.
        assert!(testlook!(look, "𝛃", 0));
        assert!(testlook!(look, "𝛃", 4));
        assert!(testlook!(look, "𝛃 ", 4));
        assert!(testlook!(look, " 𝛃 ", 1));
        assert!(testlook!(look, " 𝛃 ", 5));

        // Unicode word boundaries between non-ASCII codepoints.
        assert!(testlook!(look, "𝛃𐆀", 0));
        assert!(testlook!(look, "𝛃𐆀", 4));

        // Non word boundaries for ASCII.
        assert!(!testlook!(look, "", 0));
        assert!(!testlook!(look, "ab", 1));
        assert!(!testlook!(look, "a ", 2));
        assert!(!testlook!(look, " a ", 0));
        assert!(!testlook!(look, " a ", 3));

        // Non word boundaries with a non-ASCII codepoint.
        assert!(!testlook!(look, "𝛃b", 4));
        assert!(!testlook!(look, "𝛃 ", 5));
        assert!(!testlook!(look, " 𝛃 ", 0));
        assert!(!testlook!(look, " 𝛃 ", 6));
        assert!(!testlook!(look, "𝛃", 1));
        assert!(!testlook!(look, "𝛃", 2));
        assert!(!testlook!(look, "𝛃", 3));

        // Non word boundaries with non-ASCII codepoints.
        assert!(!testlook!(look, "𝛃𐆀", 1));
        assert!(!testlook!(look, "𝛃𐆀", 2));
        assert!(!testlook!(look, "𝛃𐆀", 3));
        assert!(!testlook!(look, "𝛃𐆀", 5));
        assert!(!testlook!(look, "𝛃𐆀", 6));
        assert!(!testlook!(look, "𝛃𐆀", 7));
        assert!(!testlook!(look, "𝛃𐆀", 8));
    }

    #[test]
    fn look_matches_word_ascii() {
        let look = Look::WordAscii;

        // \xF0\x9D\x9B\x83 = 𝛃 (in \w)
        // \xF0\x90\x86\x80 = 𐆀 (not in \w)

        // Simple ASCII word boundaries.
        assert!(testlook!(look, "a", 0));
        assert!(testlook!(look, "a", 1));
        assert!(testlook!(look, "a ", 1));
        assert!(testlook!(look, " a ", 1));
        assert!(testlook!(look, " a ", 2));

        // Unicode word boundaries with a non-ASCII codepoint. Since this is
        // an ASCII word boundary, none of these match.
        assert!(!testlook!(look, "𝛃", 0));
        assert!(!testlook!(look, "𝛃", 4));
        assert!(!testlook!(look, "𝛃 ", 4));
        assert!(!testlook!(look, " 𝛃 ", 1));
        assert!(!testlook!(look, " 𝛃 ", 5));

        // Unicode word boundaries between non-ASCII codepoints. Again, since
        // this is an ASCII word boundary, none of these match.
        assert!(!testlook!(look, "𝛃𐆀", 0));
        assert!(!testlook!(look, "𝛃𐆀", 4));

        // Non word boundaries for ASCII.
        assert!(!testlook!(look, "", 0));
        assert!(!testlook!(look, "ab", 1));
        assert!(!testlook!(look, "a ", 2));
        assert!(!testlook!(look, " a ", 0));
        assert!(!testlook!(look, " a ", 3));

        // Non word boundaries with a non-ASCII codepoint.
        assert!(testlook!(look, "𝛃b", 4));
        assert!(!testlook!(look, "𝛃 ", 5));
        assert!(!testlook!(look, " 𝛃 ", 0));
        assert!(!testlook!(look, " 𝛃 ", 6));
        assert!(!testlook!(look, "𝛃", 1));
        assert!(!testlook!(look, "𝛃", 2));
        assert!(!testlook!(look, "𝛃", 3));

        // Non word boundaries with non-ASCII codepoints.
        assert!(!testlook!(look, "𝛃𐆀", 1));
        assert!(!testlook!(look, "𝛃𐆀", 2));
        assert!(!testlook!(look, "𝛃𐆀", 3));
        assert!(!testlook!(look, "𝛃𐆀", 5));
        assert!(!testlook!(look, "𝛃𐆀", 6));
        assert!(!testlook!(look, "𝛃𐆀", 7));
        assert!(!testlook!(look, "𝛃𐆀", 8));
    }

    #[test]
    #[cfg(all(not(miri), feature = "unicode-word-boundary"))]
    fn look_matches_word_unicode_negate() {
        let look = Look::WordUnicodeNegate;

        // \xF0\x9D\x9B\x83 = 𝛃 (in \w)
        // \xF0\x90\x86\x80 = 𐆀 (not in \w)

        // Simple ASCII word boundaries.
        assert!(!testlook!(look, "a", 0));
        assert!(!testlook!(look, "a", 1));
        assert!(!testlook!(look, "a ", 1));
        assert!(!testlook!(look, " a ", 1));
        assert!(!testlook!(look, " a ", 2));

        // Unicode word boundaries with a non-ASCII codepoint.
        assert!(!testlook!(look, "𝛃", 0));
        assert!(!testlook!(look, "𝛃", 4));
        assert!(!testlook!(look, "𝛃 ", 4));
        assert!(!testlook!(look, " 𝛃 ", 1));
        assert!(!testlook!(look, " 𝛃 ", 5));

        // Unicode word boundaries between non-ASCII codepoints.
        assert!(!testlook!(look, "𝛃𐆀", 0));
        assert!(!testlook!(look, "𝛃𐆀", 4));

        // Non word boundaries for ASCII.
        assert!(testlook!(look, "", 0));
        assert!(testlook!(look, "ab", 1));
        assert!(testlook!(look, "a ", 2));
        assert!(testlook!(look, " a ", 0));
        assert!(testlook!(look, " a ", 3));

        // Non word boundaries with a non-ASCII codepoint.
        assert!(testlook!(look, "𝛃b", 4));
        assert!(testlook!(look, "𝛃 ", 5));
        assert!(testlook!(look, " 𝛃 ", 0));
        assert!(testlook!(look, " 𝛃 ", 6));
        // These don't match because they could otherwise return an offset that
        // splits the UTF-8 encoding of a codepoint.
        assert!(!testlook!(look, "𝛃", 1));
        assert!(!testlook!(look, "𝛃", 2));
        assert!(!testlook!(look, "𝛃", 3));

        // Non word boundaries with non-ASCII codepoints. These also don't
        // match because they could otherwise return an offset that splits the
        // UTF-8 encoding of a codepoint.
        assert!(!testlook!(look, "𝛃𐆀", 1));
        assert!(!testlook!(look, "𝛃𐆀", 2));
        assert!(!testlook!(look, "𝛃𐆀", 3));
        assert!(!testlook!(look, "𝛃𐆀", 5));
        assert!(!testlook!(look, "𝛃𐆀", 6));
        assert!(!testlook!(look, "𝛃𐆀", 7));
        // But this one does, since 𐆀 isn't a word codepoint, and 8 is the end
        // of the haystack. So the "end" of the haystack isn't a word and 𐆀
        // isn't a word, thus, \B matches.
        assert!(testlook!(look, "𝛃𐆀", 8));
    }

    #[test]
    fn look_matches_word_ascii_negate() {
        let look = Look::WordAsciiNegate;

        // \xF0\x9D\x9B\x83 = 𝛃 (in \w)
        // \xF0\x90\x86\x80 = 𐆀 (not in \w)

        // Simple ASCII word boundaries.
        assert!(!testlook!(look, "a", 0));
        assert!(!testlook!(look, "a", 1));
        assert!(!testlook!(look, "a ", 1));
        assert!(!testlook!(look, " a ", 1));
        assert!(!testlook!(look, " a ", 2));

        // Unicode word boundaries with a non-ASCII codepoint. Since this is
        // an ASCII word boundary, none of these match.
        assert!(testlook!(look, "𝛃", 0));
        assert!(testlook!(look, "𝛃", 4));
        assert!(testlook!(look, "𝛃 ", 4));
        assert!(testlook!(look, " 𝛃 ", 1));
        assert!(testlook!(look, " 𝛃 ", 5));

        // Unicode word boundaries between non-ASCII codepoints. Again, since
        // this is an ASCII word boundary, none of these match.
        assert!(testlook!(look, "𝛃𐆀", 0));
        assert!(testlook!(look, "𝛃𐆀", 4));

        // Non word boundaries for ASCII.
        assert!(testlook!(look, "", 0));
        assert!(testlook!(look, "ab", 1));
        assert!(testlook!(look, "a ", 2));
        assert!(testlook!(look, " a ", 0));
        assert!(testlook!(look, " a ", 3));

        // Non word boundaries with a non-ASCII codepoint.
        assert!(!testlook!(look, "𝛃b", 4));
        assert!(testlook!(look, "𝛃 ", 5));
        assert!(testlook!(look, " 𝛃 ", 0));
        assert!(testlook!(look, " 𝛃 ", 6));
        assert!(testlook!(look, "𝛃", 1));
        assert!(testlook!(look, "𝛃", 2));
        assert!(testlook!(look, "𝛃", 3));

        // Non word boundaries with non-ASCII codepoints.
        assert!(testlook!(look, "𝛃𐆀", 1));
        assert!(testlook!(look, "𝛃𐆀", 2));
        assert!(testlook!(look, "𝛃𐆀", 3));
        assert!(testlook!(look, "𝛃𐆀", 5));
        assert!(testlook!(look, "𝛃𐆀", 6));
        assert!(testlook!(look, "𝛃𐆀", 7));
        assert!(testlook!(look, "𝛃𐆀", 8));
    }

    #[test]
    fn look_matches_word_start_ascii() {
        let look = Look::WordStartAscii;

        // \xF0\x9D\x9B\x83 = 𝛃 (in \w)
        // \xF0\x90\x86\x80 = 𐆀 (not in \w)

        // Simple ASCII word boundaries.
        assert!(testlook!(look, "a", 0));
        assert!(!testlook!(look, "a", 1));
        assert!(!testlook!(look, "a ", 1));
        assert!(testlook!(look, " a ", 1));
        assert!(!testlook!(look, " a ", 2));

        // Unicode word boundaries with a non-ASCII codepoint. Since this is
        // an ASCII word boundary, none of these match.
        assert!(!testlook!(look, "𝛃", 0));
        assert!(!testlook!(look, "𝛃", 4));
        assert!(!testlook!(look, "𝛃 ", 4));
        assert!(!testlook!(look, " 𝛃 ", 1));
        assert!(!testlook!(look, " 𝛃 ", 5));

        // Unicode word boundaries between non-ASCII codepoints. Again, since
        // this is an ASCII word boundary, none of these match.
        assert!(!testlook!(look, "𝛃𐆀", 0));
        assert!(!testlook!(look, "𝛃𐆀", 4));

        // Non word boundaries for ASCII.
        assert!(!testlook!(look, "", 0));
        assert!(!testlook!(look, "ab", 1));
        assert!(!testlook!(look, "a ", 2));
        assert!(!testlook!(look, " a ", 0));
        assert!(!testlook!(look, " a ", 3));

        // Non word boundaries with a non-ASCII codepoint.
        assert!(testlook!(look, "𝛃b", 4));
        assert!(!testlook!(look, "b𝛃", 1));
        assert!(!testlook!(look, "𝛃 ", 5));
        assert!(!testlook!(look, " 𝛃 ", 0));
        assert!(!testlook!(look, " 𝛃 ", 6));
        assert!(!testlook!(look, "𝛃", 1));
        assert!(!testlook!(look, "𝛃", 2));
        assert!(!testlook!(look, "𝛃", 3));

        // Non word boundaries with non-ASCII codepoints.
        assert!(!testlook!(look, "𝛃𐆀", 1));
        assert!(!testlook!(look, "𝛃𐆀", 2));
        assert!(!testlook!(look, "𝛃𐆀", 3));
        assert!(!testlook!(look, "𝛃𐆀", 5));
        assert!(!testlook!(look, "𝛃𐆀", 6));
        assert!(!testlook!(look, "𝛃𐆀", 7));
        assert!(!testlook!(look, "𝛃𐆀", 8));
    }

    #[test]
    fn look_matches_word_end_ascii() {
        let look = Look::WordEndAscii;

        // \xF0\x9D\x9B\x83 = 𝛃 (in \w)
        // \xF0\x90\x86\x80 = 𐆀 (not in \w)

        // Simple ASCII word boundaries.
        assert!(!testlook!(look, "a", 0));
        assert!(testlook!(look, "a", 1));
        assert!(testlook!(look, "a ", 1));
        assert!(!testlook!(look, " a ", 1));
        assert!(testlook!(look, " a ", 2));

        // Unicode word boundaries with a non-ASCII codepoint. Since this is
        // an ASCII word boundary, none of these match.
        assert!(!testlook!(look, "𝛃", 0));
        assert!(!testlook!(look, "𝛃", 4));
        assert!(!testlook!(look, "𝛃 ", 4));
        assert!(!testlook!(look, " 𝛃 ", 1));
        assert!(!testlook!(look, " 𝛃 ", 5));

        // Unicode word boundaries between non-ASCII codepoints. Again, since
        // this is an ASCII word boundary, none of these match.
        assert!(!testlook!(look, "𝛃𐆀", 0));
        assert!(!testlook!(look, "𝛃𐆀", 4));

        // Non word boundaries for ASCII.
        assert!(!testlook!(look, "", 0));
        assert!(!testlook!(look, "ab", 1));
        assert!(!testlook!(look, "a ", 2));
        assert!(!testlook!(look, " a ", 0));
        assert!(!testlook!(look, " a ", 3));

        // Non word boundaries with a non-ASCII codepoint.
        assert!(!testlook!(look, "𝛃b", 4));
        assert!(testlook!(look, "b𝛃", 1));
        assert!(!testlook!(look, "𝛃 ", 5));
        assert!(!testlook!(look, " 𝛃 ", 0));
        assert!(!testlook!(look, " 𝛃 ", 6));
        assert!(!testlook!(look, "𝛃", 1));
        assert!(!testlook!(look, "𝛃", 2));
        assert!(!testlook!(look, "𝛃", 3));

        // Non word boundaries with non-ASCII codepoints.
        assert!(!testlook!(look, "𝛃𐆀", 1));
        assert!(!testlook!(look, "𝛃𐆀", 2));
        assert!(!testlook!(look, "𝛃𐆀", 3));
        assert!(!testlook!(look, "𝛃𐆀", 5));
        assert!(!testlook!(look, "𝛃𐆀", 6));
        assert!(!testlook!(look, "𝛃𐆀", 7));
        assert!(!testlook!(look, "𝛃𐆀", 8));
    }

    #[test]
    #[cfg(all(not(miri), feature = "unicode-word-boundary"))]
    fn look_matches_word_start_unicode() {
        let look = Look::WordStartUnicode;

        // \xF0\x9D\x9B\x83 = 𝛃 (in \w)
        // \xF0\x90\x86\x80 = 𐆀 (not in \w)

        // Simple ASCII word boundaries.
        assert!(testlook!(look, "a", 0));
        assert!(!testlook!(look, "a", 1));
        assert!(!testlook!(look, "a ", 1));
        assert!(testlook!(look, " a ", 1));
        assert!(!testlook!(look, " a ", 2));

        // Unicode word boundaries with a non-ASCII codepoint.
        assert!(testlook!(look, "𝛃", 0));
        assert!(!testlook!(look, "𝛃", 4));
        assert!(!testlook!(look, "𝛃 ", 4));
        assert!(testlook!(look, " 𝛃 ", 1));
        assert!(!testlook!(look, " 𝛃 ", 5));

        // Unicode word boundaries between non-ASCII codepoints.
        assert!(testlook!(look, "𝛃𐆀", 0));
        assert!(!testlook!(look, "𝛃𐆀", 4));

        // Non word boundaries for ASCII.
        assert!(!testlook!(look, "", 0));
        assert!(!testlook!(look, "ab", 1));
        assert!(!testlook!(look, "a ", 2));
        assert!(!testlook!(look, " a ", 0));
        assert!(!testlook!(look, " a ", 3));

        // Non word boundaries with a non-ASCII codepoint.
        assert!(!testlook!(look, "𝛃b", 4));
        assert!(!testlook!(look, "b𝛃", 1));
        assert!(!testlook!(look, "𝛃 ", 5));
        assert!(!testlook!(look, " 𝛃 ", 0));
        assert!(!testlook!(look, " 𝛃 ", 6));
        assert!(!testlook!(look, "𝛃", 1));
        assert!(!testlook!(look, "𝛃", 2));
        assert!(!testlook!(look, "𝛃", 3));

        // Non word boundaries with non-ASCII codepoints.
        assert!(!testlook!(look, "𝛃𐆀", 1));
        assert!(!testlook!(look, "𝛃𐆀", 2));
        assert!(!testlook!(look, "𝛃𐆀", 3));
        assert!(!testlook!(look, "𝛃𐆀", 5));
        assert!(!testlook!(look, "𝛃𐆀", 6));
        assert!(!testlook!(look, "𝛃𐆀", 7));
        assert!(!testlook!(look, "𝛃𐆀", 8));
    }

    #[test]
    #[cfg(all(not(miri), feature = "unicode-word-boundary"))]
    fn look_matches_word_end_unicode() {
        let look = Look::WordEndUnicode;

        // \xF0\x9D\x9B\x83 = 𝛃 (in \w)
        // \xF0\x90\x86\x80 = 𐆀 (not in \w)

        // Simple ASCII word boundaries.
        assert!(!testlook!(look, "a", 0));
        assert!(testlook!(look, "a", 1));
        assert!(testlook!(look, "a ", 1));
        assert!(!testlook!(look, " a ", 1));
        assert!(testlook!(look, " a ", 2));

        // Unicode word boundaries with a non-ASCII codepoint.
        assert!(!testlook!(look, "𝛃", 0));
        assert!(testlook!(look, "𝛃", 4));
        assert!(testlook!(look, "𝛃 ", 4));
        assert!(!testlook!(look, " 𝛃 ", 1));
        assert!(testlook!(look, " 𝛃 ", 5));

        // Unicode word boundaries between non-ASCII codepoints.
        assert!(!testlook!(look, "𝛃𐆀", 0));
        assert!(testlook!(look, "𝛃𐆀", 4));

        // Non word boundaries for ASCII.
        assert!(!testlook!(look, "", 0));
        assert!(!testlook!(look, "ab", 1));
        assert!(!testlook!(look, "a ", 2));
        assert!(!testlook!(look, " a ", 0));
        assert!(!testlook!(look, " a ", 3));

        // Non word boundaries with a non-ASCII codepoint.
        assert!(!testlook!(look, "𝛃b", 4));
        assert!(!testlook!(look, "b𝛃", 1));
        assert!(!testlook!(look, "𝛃 ", 5));
        assert!(!testlook!(look, " 𝛃 ", 0));
        assert!(!testlook!(look, " 𝛃 ", 6));
        assert!(!testlook!(look, "𝛃", 1));
        assert!(!testlook!(look, "𝛃", 2));
        assert!(!testlook!(look, "𝛃", 3));

        // Non word boundaries with non-ASCII codepoints.
        assert!(!testlook!(look, "𝛃𐆀", 1));
        assert!(!testlook!(look, "𝛃𐆀", 2));
        assert!(!testlook!(look, "𝛃𐆀", 3));
        assert!(!testlook!(look, "𝛃𐆀", 5));
        assert!(!testlook!(look, "𝛃𐆀", 6));
        assert!(!testlook!(look, "𝛃𐆀", 7));
        assert!(!testlook!(look, "𝛃𐆀", 8));
    }

    #[test]
    fn look_matches_word_start_half_ascii() {
        let look = Look::WordStartHalfAscii;

        // \xF0\x9D\x9B\x83 = 𝛃 (in \w)
        // \xF0\x90\x86\x80 = 𐆀 (not in \w)

        // Simple ASCII word boundaries.
        assert!(testlook!(look, "a", 0));
        assert!(!testlook!(look, "a", 1));
        assert!(!testlook!(look, "a ", 1));
        assert!(testlook!(look, " a ", 1));
        assert!(!testlook!(look, " a ", 2));

        // Unicode word boundaries with a non-ASCII codepoint. Since this is
        // an ASCII word boundary, none of these match.
        assert!(testlook!(look, "𝛃", 0));
        assert!(testlook!(look, "𝛃", 4));
        assert!(testlook!(look, "𝛃 ", 4));
        assert!(testlook!(look, " 𝛃 ", 1));
        assert!(testlook!(look, " 𝛃 ", 5));

        // Unicode word boundaries between non-ASCII codepoints. Again, since
        // this is an ASCII word boundary, none of these match.
        assert!(testlook!(look, "𝛃𐆀", 0));
        assert!(testlook!(look, "𝛃𐆀", 4));

        // Non word boundaries for ASCII.
        assert!(testlook!(look, "", 0));
        assert!(!testlook!(look, "ab", 1));
        assert!(testlook!(look, "a ", 2));
        assert!(testlook!(look, " a ", 0));
        assert!(testlook!(look, " a ", 3));

        // Non word boundaries with a non-ASCII codepoint.
        assert!(testlook!(look, "𝛃b", 4));
        assert!(!testlook!(look, "b𝛃", 1));
        assert!(testlook!(look, "𝛃 ", 5));
        assert!(testlook!(look, " 𝛃 ", 0));
        assert!(testlook!(look, " 𝛃 ", 6));
        assert!(testlook!(look, "𝛃", 1));
        assert!(testlook!(look, "𝛃", 2));
        assert!(testlook!(look, "𝛃", 3));

        // Non word boundaries with non-ASCII codepoints.
        assert!(testlook!(look, "𝛃𐆀", 1));
        assert!(testlook!(look, "𝛃𐆀", 2));
        assert!(testlook!(look, "𝛃𐆀", 3));
        assert!(testlook!(look, "𝛃𐆀", 5));
        assert!(testlook!(look, "𝛃𐆀", 6));
        assert!(testlook!(look, "𝛃𐆀", 7));
        assert!(testlook!(look, "𝛃𐆀", 8));
    }

    #[test]
    fn look_matches_word_end_half_ascii() {
        let look = Look::WordEndHalfAscii;

        // \xF0\x9D\x9B\x83 = 𝛃 (in \w)
        // \xF0\x90\x86\x80 = 𐆀 (not in \w)

        // Simple ASCII word boundaries.
        assert!(!testlook!(look, "a", 0));
        assert!(testlook!(look, "a", 1));
        assert!(testlook!(look, "a ", 1));
        assert!(!testlook!(look, " a ", 1));
        assert!(testlook!(look, " a ", 2));

        // Unicode word boundaries with a non-ASCII codepoint. Since this is
        // an ASCII word boundary, none of these match.
        assert!(testlook!(look, "𝛃", 0));
        assert!(testlook!(look, "𝛃", 4));
        assert!(testlook!(look, "𝛃 ", 4));
        assert!(testlook!(look, " 𝛃 ", 1));
        assert!(testlook!(look, " 𝛃 ", 5));

        // Unicode word boundaries between non-ASCII codepoints. Again, since
        // this is an ASCII word boundary, none of these match.
        assert!(testlook!(look, "𝛃𐆀", 0));
        assert!(testlook!(look, "𝛃𐆀", 4));

        // Non word boundaries for ASCII.
        assert!(testlook!(look, "", 0));
        assert!(!testlook!(look, "ab", 1));
        assert!(testlook!(look, "a ", 2));
        assert!(testlook!(look, " a ", 0));
        assert!(testlook!(look, " a ", 3));

        // Non word boundaries with a non-ASCII codepoint.
        assert!(!testlook!(look, "𝛃b", 4));
        assert!(testlook!(look, "b𝛃", 1));
        assert!(testlook!(look, "𝛃 ", 5));
        assert!(testlook!(look, " 𝛃 ", 0));
        assert!(testlook!(look, " 𝛃 ", 6));
        assert!(testlook!(look, "𝛃", 1));
        assert!(testlook!(look, "𝛃", 2));
        assert!(testlook!(look, "𝛃", 3));

        // Non word boundaries with non-ASCII codepoints.
        assert!(testlook!(look, "𝛃𐆀", 1));
        assert!(testlook!(look, "𝛃𐆀", 2));
        assert!(testlook!(look, "𝛃𐆀", 3));
        assert!(testlook!(look, "𝛃𐆀", 5));
        assert!(testlook!(look, "𝛃𐆀", 6));
        assert!(testlook!(look, "𝛃𐆀", 7));
        assert!(testlook!(look, "𝛃𐆀", 8));
    }

    #[test]
    #[cfg(all(not(miri), feature = "unicode-word-boundary"))]
    fn look_matches_word_start_half_unicode() {
        let look = Look::WordStartHalfUnicode;

        // \xF0\x9D\x9B\x83 = 𝛃 (in \w)
        // \xF0\x90\x86\x80 = 𐆀 (not in \w)

        // Simple ASCII word boundaries.
        assert!(testlook!(look, "a", 0));
        assert!(!testlook!(look, "a", 1));
        assert!(!testlook!(look, "a ", 1));
        assert!(testlook!(look, " a ", 1));
        assert!(!testlook!(look, " a ", 2));

        // Unicode word boundaries with a non-ASCII codepoint.
        assert!(testlook!(look, "𝛃", 0));
        assert!(!testlook!(look, "𝛃", 4));
        assert!(!testlook!(look, "𝛃 ", 4));
        assert!(testlook!(look, " 𝛃 ", 1));
        assert!(!testlook!(look, " 𝛃 ", 5));

        // Unicode word boundaries between non-ASCII codepoints.
        assert!(testlook!(look, "𝛃𐆀", 0));
        assert!(!testlook!(look, "𝛃𐆀", 4));

        // Non word boundaries for ASCII.
        assert!(testlook!(look, "", 0));
        assert!(!testlook!(look, "ab", 1));
        assert!(testlook!(look, "a ", 2));
        assert!(testlook!(look, " a ", 0));
        assert!(testlook!(look, " a ", 3));

        // Non word boundaries with a non-ASCII codepoint.
        assert!(!testlook!(look, "𝛃b", 4));
        assert!(!testlook!(look, "b𝛃", 1));
        assert!(testlook!(look, "𝛃 ", 5));
        assert!(testlook!(look, " 𝛃 ", 0));
        assert!(testlook!(look, " 𝛃 ", 6));
        assert!(!testlook!(look, "𝛃", 1));
        assert!(!testlook!(look, "𝛃", 2));
        assert!(!testlook!(look, "𝛃", 3));

        // Non word boundaries with non-ASCII codepoints.
        assert!(!testlook!(look, "𝛃𐆀", 1));
        assert!(!testlook!(look, "𝛃𐆀", 2));
        assert!(!testlook!(look, "𝛃𐆀", 3));
        assert!(!testlook!(look, "𝛃𐆀", 5));
        assert!(!testlook!(look, "𝛃𐆀", 6));
        assert!(!testlook!(look, "𝛃𐆀", 7));
        assert!(testlook!(look, "𝛃𐆀", 8));
    }

    #[test]
    #[cfg(all(not(miri), feature = "unicode-word-boundary"))]
    fn look_matches_word_end_half_unicode() {
        let look = Look::WordEndHalfUnicode;

        // \xF0\x9D\x9B\x83 = 𝛃 (in \w)
        // \xF0\x90\x86\x80 = 𐆀 (not in \w)

        // Simple ASCII word boundaries.
        assert!(!testlook!(look, "a", 0));
        assert!(testlook!(look, "a", 1));
        assert!(testlook!(look, "a ", 1));
        assert!(!testlook!(look, " a ", 1));
        assert!(testlook!(look, " a ", 2));

        // Unicode word boundaries with a non-ASCII codepoint.
        assert!(!testlook!(look, "𝛃", 0));
        assert!(testlook!(look, "𝛃", 4));
        assert!(testlook!(look, "𝛃 ", 4));
        assert!(!testlook!(look, " 𝛃 ", 1));
        assert!(testlook!(look, " 𝛃 ", 5));

        // Unicode word boundaries between non-ASCII codepoints.
        assert!(!testlook!(look, "𝛃𐆀", 0));
        assert!(testlook!(look, "𝛃𐆀", 4));

        // Non word boundaries for ASCII.
        assert!(testlook!(look, "", 0));
        assert!(!testlook!(look, "ab", 1));
        assert!(testlook!(look, "a ", 2));
        assert!(testlook!(look, " a ", 0));
        assert!(testlook!(look, " a ", 3));

        // Non word boundaries with a non-ASCII codepoint.
        assert!(!testlook!(look, "𝛃b", 4));
        assert!(!testlook!(look, "b𝛃", 1));
        assert!(testlook!(look, "𝛃 ", 5));
        assert!(testlook!(look, " 𝛃 ", 0));
        assert!(testlook!(look, " 𝛃 ", 6));
        assert!(!testlook!(look, "𝛃", 1));
        assert!(!testlook!(look, "𝛃", 2));
        assert!(!testlook!(look, "𝛃", 3));

        // Non word boundaries with non-ASCII codepoints.
        assert!(!testlook!(look, "𝛃𐆀", 1));
        assert!(!testlook!(look, "𝛃𐆀", 2));
        assert!(!testlook!(look, "𝛃𐆀", 3));
        assert!(!testlook!(look, "𝛃𐆀", 5));
        assert!(!testlook!(look, "𝛃𐆀", 6));
        assert!(!testlook!(look, "𝛃𐆀", 7));
        assert!(testlook!(look, "𝛃𐆀", 8));
    }

    #[test]
    fn look_set() {
        let mut f = LookSet::default();
        assert!(!f.contains(Look::Start));
        assert!(!f.contains(Look::End));
        assert!(!f.contains(Look::StartLF));
        assert!(!f.contains(Look::EndLF));
        assert!(!f.contains(Look::WordUnicode));
        assert!(!f.contains(Look::WordUnicodeNegate));
        assert!(!f.contains(Look::WordAscii));
        assert!(!f.contains(Look::WordAsciiNegate));

        f = f.insert(Look::Start);
        assert!(f.contains(Look::Start));
        f = f.remove(Look::Start);
        assert!(!f.contains(Look::Start));

        f = f.insert(Look::End);
        assert!(f.contains(Look::End));
        f = f.remove(Look::End);
        assert!(!f.contains(Look::End));

        f = f.insert(Look::StartLF);
        assert!(f.contains(Look::StartLF));
        f = f.remove(Look::StartLF);
        assert!(!f.contains(Look::StartLF));

        f = f.insert(Look::EndLF);
        assert!(f.contains(Look::EndLF));
        f = f.remove(Look::EndLF);
        assert!(!f.contains(Look::EndLF));

        f = f.insert(Look::StartCRLF);
        assert!(f.contains(Look::StartCRLF));
        f = f.remove(Look::StartCRLF);
        assert!(!f.contains(Look::StartCRLF));

        f = f.insert(Look::EndCRLF);
        assert!(f.contains(Look::EndCRLF));
        f = f.remove(Look::EndCRLF);
        assert!(!f.contains(Look::EndCRLF));

        f = f.insert(Look::WordUnicode);
        assert!(f.contains(Look::WordUnicode));
        f = f.remove(Look::WordUnicode);
        assert!(!f.contains(Look::WordUnicode));

        f = f.insert(Look::WordUnicodeNegate);
        assert!(f.contains(Look::WordUnicodeNegate));
        f = f.remove(Look::WordUnicodeNegate);
        assert!(!f.contains(Look::WordUnicodeNegate));

        f = f.insert(Look::WordAscii);
        assert!(f.contains(Look::WordAscii));
        f = f.remove(Look::WordAscii);
        assert!(!f.contains(Look::WordAscii));

        f = f.insert(Look::WordAsciiNegate);
        assert!(f.contains(Look::WordAsciiNegate));
        f = f.remove(Look::WordAsciiNegate);
        assert!(!f.contains(Look::WordAsciiNegate));

        f = f.insert(Look::WordStartAscii);
        assert!(f.contains(Look::WordStartAscii));
        f = f.remove(Look::WordStartAscii);
        assert!(!f.contains(Look::WordStartAscii));

        f = f.insert(Look::WordEndAscii);
        assert!(f.contains(Look::WordEndAscii));
        f = f.remove(Look::WordEndAscii);
        assert!(!f.contains(Look::WordEndAscii));

        f = f.insert(Look::WordStartUnicode);
        assert!(f.contains(Look::WordStartUnicode));
        f = f.remove(Look::WordStartUnicode);
        assert!(!f.contains(Look::WordStartUnicode));

        f = f.insert(Look::WordEndUnicode);
        assert!(f.contains(Look::WordEndUnicode));
        f = f.remove(Look::WordEndUnicode);
        assert!(!f.contains(Look::WordEndUnicode));

        f = f.insert(Look::WordStartHalfAscii);
        assert!(f.contains(Look::WordStartHalfAscii));
        f = f.remove(Look::WordStartHalfAscii);
        assert!(!f.contains(Look::WordStartHalfAscii));

        f = f.insert(Look::WordEndHalfAscii);
        assert!(f.contains(Look::WordEndHalfAscii));
        f = f.remove(Look::WordEndHalfAscii);
        assert!(!f.contains(Look::WordEndHalfAscii));

        f = f.insert(Look::WordStartHalfUnicode);
        assert!(f.contains(Look::WordStartHalfUnicode));
        f = f.remove(Look::WordStartHalfUnicode);
        assert!(!f.contains(Look::WordStartHalfUnicode));

        f = f.insert(Look::WordEndHalfUnicode);
        assert!(f.contains(Look::WordEndHalfUnicode));
        f = f.remove(Look::WordEndHalfUnicode);
        assert!(!f.contains(Look::WordEndHalfUnicode));
    }

    #[test]
    fn look_set_iter() {
        let set = LookSet::empty();
        assert_eq!(0, set.iter().count());

        let set = LookSet::full();
        assert_eq!(18, set.iter().count());

        let set =
            LookSet::empty().insert(Look::StartLF).insert(Look::WordUnicode);
        assert_eq!(2, set.iter().count());

        let set = LookSet::empty().insert(Look::StartLF);
        assert_eq!(1, set.iter().count());

        let set = LookSet::empty().insert(Look::WordAsciiNegate);
        assert_eq!(1, set.iter().count());

        let set = LookSet::empty().insert(Look::WordEndHalfUnicode);
        assert_eq!(1, set.iter().count());
    }

    #[test]
    #[cfg(feature = "alloc")]
    fn look_set_debug() {
        let res = alloc::format!("{:?}", LookSet::empty());
        assert_eq!("∅", res);
        let res = alloc::format!("{:?}", LookSet::full());
        assert_eq!("Az^$rRbB𝛃𝚩<>〈〉◁▷◀▶", res);
    }
}