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
|
# Regex builder DSL
* Proposal: [SE-NNNN](NNNN-filename.md)
* Authors: [Richard Wei](https://github.com/rxwei), [Michael Ilseman](https://github.com/milseman), [Nate Cook](https://github.com/natecook1000)
* Review Manager: TBD
* Implementation: [apple/swift-experimental-string-processing](https://github.com/apple/swift-experimental-string-processing/tree/main/Sources/_StringProcessing/RegexDSL)
* Status: **Pitch**
**Table of Contents**
- [Introduction](#introduction)
- [Motivation](#motivation)
- [Proposed solution](#proposed-solution)
- [Detailed design](#detailed-design)
- [`RegexComponent` protocol](#regexcomponent-protocol)
- [Concatenation](#concatenation)
- [Alternation](#alternation)
- [Quantification](#quantification)
- [Capture and reference](#capture-and-reference)
- [Subpattern](#subpattern)
- [Scoping](#scoping)
- [Source compatibility](#source-compatibility)
- [Effect on ABI stability](#effect-on-abi-stability)
- [Effect on API resilience](#effect-on-api-resilience)
- [Alternatives considered](#alternatives-considered)
## Introduction
[Declarative string processing] aims to offer powerful pattern matching capabilities with expressivity, clarity, type safety, and ease of use. To achieve this, we propose to introduce a result-builder-based DSL, **regex builder**, for creating and composing regular expressions (**regex**es).
Regex builder is part of the Swift Standard Library but resides in a standalone module named `RegexBuilder`. By importing `RegexBuilder`, you get all necessary API for building a regex.
```swift
import RegexBuilder
let emailPattern = Regex {
let word = OneOrMore(.word)
Capture {
ZeroOrMore {
word
"."
}
word
}
"@"
Capture {
word
OneOrMore {
"."
word
}
}
} // => Regex<(Substring, Substring, Substring)>
let email = "My email is my.name@mail.swift.org."
if let match = email.firstMatch(of: emailPattern) {
let (wholeMatch, name, domain) = match.output
// wholeMatch: "My email is my.name@mail.swift.org."
// name: "my.name"
// domain: "mail.swift.org"
}
```
This proposal introduces all core API for creating and composing regexes that echos the textual [regex syntax] and [strongly typed regex captures], but does not formally specify the matching semantics or define character classes.
## Motivation
Regex is a fundemental and powerful tool for textual pattern matching. It is a domain-specific language often expressed as text. For example, given the following bank statement:
```
CREDIT 04062020 PayPal transfer $4.99
CREDIT 04032020 Payroll $69.73
DEBIT 04022020 ACH transfer $38.25
DEBIT 03242020 IRS tax payment $52249.98
```
One can write the follow textual regex to match each line:
```
(CREDIT|DEBIT)\s+(\d{2}\d{2}\d{4})\s+([\w\s]+\w)\s+(\$\d+\.\d{2})
```
While a regex like this is very compact and expressive, it is very difficult read, write and use:
1. Syntactic special characters, e.g. `\`, `(`, `[`, `{`, are too dense to be readable.
2. It contains a hierarchy of subpatterns fit into a single line of text.
3. No code completion when typing syntactic components.
4. Capturing groups produce raw data (i.e. a range or a substring) and can only be converted to other data structures after matching.
5. While comments `(?#...)` can be added inline, it only complicates readability.
## Proposed solution
We introduce regex builder, a result-builder-based API for creating and composing regexes. This API resides in a new module named `RegexBuilder` that is to be shipped as part of the Swift toolchain.
With regex builder, the regex for matching a bank statement can be written as the following:
```swift
import RegexBuilder
enum TransactionKind: String {
case credit = "CREDIT"
case debit = "DEBIT"
}
struct Date {
var month, day, year: Int
init?(mmddyyyy: String) { ... }
}
struct Amount {
var valueTimes100: Int
init?(twoDecimalPlaces text: Substring) { ... }
}
let statementPattern = Regex {
// Parse the transaction kind.
TryCapture {
ChoiceOf {
"CREDIT"
"DEBIT"
}
} transform: {
TransactionKind(rawValue: String($0))
}
OneOrMore(.whitespace)
// Parse the date, e.g. "01012021".
TryCapture {
Repeat(.digit, count: 2)
Repeat(.digit, count: 2)
Repeat(.digit, count: 4)
} transform: { Date(mmddyyyy: $0) }
OneOrMore(.whitespace)
// Parse the transaction description, e.g. "ACH transfer".
Capture {
OneOrMore(.custom([
.characterClass(.word),
.characterClass(.whitespace)
]))
CharacterClass.word
} transform: { String($0) }
OneOrMore(.whitespace)
"$"
// Parse the amount, e.g. `$100.00`.
TryCapture {
OneOrMore(.digit)
"."
Repeat(.digit, count: 2)
} transform: { Amount(twoDecimalPlaces: $0) }
} // => Regex<(Substring, TransactionKind, Date, String, Amount)>
let statement = """
CREDIT 04062020 PayPal transfer $4.99
CREDIT 04032020 Payroll $69.73
DEBIT 04022020 ACH transfer $38.25
DEBIT 03242020 IRS tax payment $52249.98
"""
for match in statement.matches(of: statementPattern) {
let (line, kind, date, description, amount) = match.output
...
}
```
Regex builder addresses all of textual regexes' shortcomings presented in the [Motivation](#motivation) section:
1. Capture groups and quantifiers are expressed as API calls that are easy to read.
2. Scoping and indentations clearly distinguish subpatterns in the hierarchy.
3. Code completion is available when the developer types an API call.
4. Capturing groups can be transformed into structured data at the regex declaration site.
5. Normal code comments can be written within a regex declaration to further improve readability.
## Detailed design
### `RegexComponent` protocol
One of the goals of the regex builder DSL is allowing the developers to easily compose regexes from common currency types and literals, or even define custom patterns to use for matching. We introduce `RegexComponent`, a protocol that unifies all types that can represent a component of a regex.
```swift
public protocol RegexComponent {
associatedtype Output
@RegexComponentBuilder
var regex: Regex<Output> { get }
}
```
By conforming standard library types to `RegexComponent`, we allow them to be used inside the regex builder DSL as a match target.
```swift
// A string represents a regex that matches the string.
extension String: RegexComponent {
public var regex: Regex<Substring> { get }
}
// A substring represents a regex that matches the substring.
extension Substring: RegexComponent {
public var regex: Regex<Substring> { get }
}
// A character represents a regex that matches the character.
extension Character: RegexComponent {
public var regex: Regex<Substring> { get }
}
// A unicode scalar represents a regex that matches the scalar.
extension UnicodeScalar: RegexComponent {
public var regex: Regex<Substring> { get }
}
// To be introduced in a future pitch.
extension CharacterClass: RegexComponent {
public var regex: Regex<Substring> { get }
}
```
Since regexes are composable, the `Regex` type itself also conforms to `RegexComponent`.
```swift
extension Regex: RegexComponent {
public var regex: Self { self }
}
```
All of the regex builder DSL in the rest of this pitch will accept generic components that conform to `RegexComponent`.
### Concatenation
A regex can be viewed as a concatenation of smaller regexes. In the regex builder DSL, `RegexComponentBuilder` is the basic facility to allow developers to compose regexes by concatenation.
```swift
@resultBuilder
public enum RegexComponentBuilder { ... }
```
A closure marked with `@RegexComponentBuilder` will be transformed to produce a `Regex` by concatenating all of its components, where the result type's `Output` type will be a `Substring` followed by concatenated captures (tuple when plural).
> #### Recap: Regex capturing basics
>
> `Regex` is a generic type with generic parameter `Output`.
>
> ```swift
> struct Regex<Output> { ... }
> ```
>
> When a regex does not contain any capturing groups, its `Output` type is `Substring`, which represents the whole matched portion of the input.
>
> ```swift
> let noCaptures = #/a/# // => Regex<Substring>
> ```
>
> When a regex contains capturing groups, i.e. `(...)`, the `Output` type is extended as a tuple to also contain *capture types*. Capture types are tuple elements after the first element.
>
> ```swift
> // ________________________________
> // .0 | .0 |
> // ____________________ _________
> let yesCaptures = #/a(?:(b+)c(d+))+e(f)?/# // => Regex<(Substring, Substring, Substring, Substring?)>
> // ---- ---- --- --------- --------- ----------
> // .1 | .2 | .3 | .1 | .2 | .3 |
> // | | | | | |
> // | | |_______________________________ | ______ | ________|
> // | | | |
> // | |______________________________________ | ______ |
> // | |
> // |_____________________________________________|
> // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> // Capture types
> ```
We introduce a new initializer `Regex.init(_:)` which accepts a `@RegexComponentBuilder` closure. This initializer is the entry point for creating a regex using the regex builder DSL.
```swift
extension Regex {
public init<R: RegexComponent>(
@RegexComponentBuilder _ content: () -> R
) where R.Output == Output
}
```
Example:
```swift
Regex {
regex0 // Regex<Substring>
regex1 // Regex<(Substring, Int)>
if condition {
regex2 // Regex<(Substring, Float)>
} else {
regex3 // Regex<(Substring, Substring)>
}
} // Regex<(Substring, Int, Float, Substring)>
```
This above regex will be transformed to:
```swift
Regex {
let e0 = RegexComponentBuilder.buildExpression(regex0) // Regex<Substring>
let e1 = RegexComponentBuilder.buildExpression(regex1) // Regex<(Substring, Int)>
let e2: Regex<Substring>
if condition {
e2 = RegexComponentBuilder.buildEither(first: regex2) // Regex<(Substring, Float)>
} else {
e2 = RegexComponentBuilder.buildEither(second: regex3) // Regex<(Substring, Substring)>
}
let r0 = RegexComponentBuilder.buildPartialBlock(first: e0)
let r1 = RegexComponentBuilder.buildPartialBlock(accumulated: r0, next: r1)
let r2 = RegexComponentBuilder.buildPartialBlock(accumulated: r1, next: r2)
return r2
} // Regex<(Substring, Int, Float, Substring)>
```
Basic methods in `RegexComponentBuilder`, e.g. `buildBlock()`, provides support for creating the most fundamental blocks. The `buildExpression` method wraps a user-provided component in a `RegexComponentBuilder.Component` structure, before passing the component to other builder methods. This is used for saving the source location of the component so that runtime errors can be reported with an accurate location.
```swift
@resultBuilder
public enum RegexComponentBuilder {
/// Returns an empty regex.
public static func buildBlock() -> Regex<Substring>
/// A builder component that stores a regex component and its source location
/// for debugging purposes.
public struct Component<Value: RegexComponent> {
public var value: Value
public var file: String
public var function: String
public var line: Int
public var column: Int
}
/// Returns a component by wrapping the component regex in `Component` and
/// recording its source location.
public static func buildExpression<R: RegexComponent>(
_ regex: R,
file: String = #file,
function: String = #function,
line: Int = #line,
column: Int = #column
) -> Component<R>
}
```
When it comes to concatenation, `RegexComponentBuilder` utilizes the [recently proposed `buildPartialBlock` feature](https://forums.swift.org/t/pitch-buildpartialblock-for-result-builders/55561/1) to be able to concatenate all components' capture types to a single result tuple. `buildPartialBlock(first:)` provides support for creating a regex from a single component, and `buildPartialBlock(accumulated:next:)` support for creating a regex from multiple results.
Before Swift supports variadic generics, `buildPartialBlock(first:)` and `buildPartialBlock(accumulated:next:)` must be overloaded to support concatenating regexes of supported capture quantities (arities).
- `buildPartialBlock(first:)` is overloaded `arity` times such that a unary block with a component of any supported capture arity will produce a regex with capture type `Substring` followed by the component's capture types. The base overload, `buildPartialBlock<R>(first:) -> Regex<Substring>`, must be marked with `@_disfavoredOverload` to prevent it from shadowing other overloads.
- `buildPartialBlock(accumulated:next:)` is overloaded up to `arity^2` times to account for all possible pairs of regexes that make up 10 captures.
In the initial version of the DSL, we plan to support regexes with up to 10 captures, as 10 captures are sufficient for most use cases. These overloads can be superceded by variadic versions of `buildPartialBlock(first:)` and `buildPartialBlock(accumulated:next:)` in a future release.
```swift
extension RegexComponentBuilder {
// The following builder methods implement what would be possible with
// variadic generics (using imaginary syntax) as a single method:
//
// public static func buildPartialBlock<
// R, WholeMatch, Capture...
// >(
// first component: Component<R>
// ) -> Regex<(Substring, Capture...)>
// where Component.Output == (WholeMatch, Capture...),
@_disfavoredOverload
public static func buildPartialBlock<R: RegexComponent>(
first r: Component<R>
) -> Regex<Substring>
public static func buildPartialBlock<W, C0, R: RegexComponent>(
first r: Component<R>
) -> Regex<(Substring, C0)> where R.Output == (W, C0)
public static func buildPartialBlock<W, C0, C1, R: RegexComponent>(
first r: Component<R>
) -> Regex<(Substring, C0, C1)> where R.Output == (W, C0, C1)
// ... `O(arity)` overloads of `buildPartialBlock(first:)`
// The following builder methods implement what would be possible with
// variadic generics (using imaginary syntax) as a single method:
//
// public static func buildPartialBlock<
// AccumulatedWholeMatch, NextWholeMatch,
// AccumulatedCapture..., NextCapture...,
// Accumulated: RegexComponent, Next: RegexComponent
// >(
// accumulated: Accumulated, next: Component<Next>
// ) -> Regex<(Substring, AccumulatedCapture..., NextCapture...)>
// where Accumulated.Output == (AccumulatedWholeMatch, AccumulatedCapture...),
// Next.Output == (NextWholeMatch, NextCapture...)
public static func buildPartialBlock<W0, W1, C0, R0: RegexComponent, R1: RegexComponent>(
accumulated: R0, next: Component<R1>
) -> Regex<(Substring, C0)> where R0.Output == W0, R1.Output == (W1, C0)
public static func buildPartialBlock<W0, W1, C0, C1, R0: RegexComponent, R1: RegexComponent>(
accumulated: R0, next: Component<R1>
) -> Regex<(Substring, C0, C1)> where R0.Output == W0, R1.Output == (W1, C0, C1)
public static func buildPartialBlock<W0, W1, C0, C1, C2, R0: RegexComponent, R1: RegexComponent>(
accumulated: R0, next: Component<R1>
) -> Regex<(Substring, C0, C1, C2)> where R0.Output == W0, R1.Output == (W1, C0, C1, C2)
// ... `O(arity^2)` overloads of `buildPartialBlock(accumulated:next:)`
}
```
To support `if #available(...)` statements, `buildLimitedAvailability(_:)` is defined with overloads to support up to 10 captures. The overload for non-capturing regexes, due to the lack of generic constraints, must be annotated with `@_disfavoredOverload` in order not shadow other overloads. We expect that a variadic-generic version of this method will eventually superseded all of these overloads.
```swift
extension RegexComponentBuilder {
// The following builder methods implement what would be possible with
// variadic generics (using imaginary syntax) as a single method:
//
// public static func buildLimitedAvailability<
// Component, WholeMatch, Capture...
// >(
// _ component: Component
// ) where Component.Output == (WholeMatch, Capture...)
@_disfavoredOverload
public static func buildLimitedAvailability<R: RegexComponent>(
_ component: Component<R>
) -> Regex<Substring>
public static func buildLimitedAvailability<W, C0, R: RegexComponent>(
_ component: Component<R>
) -> Regex<(Substring, C0?)>
public static func buildLimitedAvailability<W, C0, C1, R: RegexComponent>(
_ component: Component<R>
) -> Regex<(Substring, C0?, C1?)>
// ... `O(arity)` overloads of `buildLimitedAvailability(_:)`
}
```
`buildOptional` and `buildEither` are intentionally not supported due to ergonomic issues and fundamental semantic differences between regex conditionals and result builder conditionals. Please refer to the [alternatives considered](#support-buildoptional-and-buildeither) section for detailed rationale.
### Alternation
Alternations are used to match one of multiple patterns. An alternation wraps its underlying patterns' capture types in an `Optional` and concatenates them together, first to last.
```swift
let choice = ChoiceOf {
regex1 // Regex<(Substring, Int)>
regex2 // Regex<(Substring, Float)>
regex3 // Regex<(Substring, Substring)>
regex0 // Regex<Substring>
} // => Regex<(Substring, Int?, Float?, Substring?)>
```
`AlternationBuilder` is a result builder type for creating alternations from components of a block.
```swift
@resultBuilder
public struct AlternationBuilder { ... }
```
To the developer, the top-level API is a type named `ChoiceOf`. This type has an initializer that accepts an `@AlternationBuilder` closure.
```swift
public struct ChoiceOf<Output>: RegexComponent {
public var regex: Regex<Output> { get }
public init<R: RegexComponent>(
@AlternationBuilder builder: () -> R
) where R.Output == Output
}
```
`AlternationBuilder` is mostly similar to `RegexComponent` with the following distinctions:
- Empty blocks are not supported.
- Capture types are wrapped in a layer of `Optional` before being concatenated in the resulting `Output` type.
- `buildEither(first:)` and `buildEither(second:)` are overloaded for each supported capture arity because they need to wrap capture types in `Optional`.
```swift
@resultBuilder
public enum AlternationBuilder {
public typealias Component<Value> = RegexComponentBuilder.Component<Value>
/// Returns a component by wrapping the component regex in `Component` and
/// recording its source location.
public static func buildExpression<R: RegexComponent>(
_ regex: R,
file: String = #file,
function: String = #function,
line: Int = #line,
column: Int = #column
) -> Component<R>
// The following builder methods implement what would be possible with
// variadic generics (using imaginary syntax) as a single method:
//
// public static func buildPartialBlock<
// R, WholeMatch, Capture...
// >(
// first component: Component<R>
// ) -> Regex<(Substring, Capture?...)>
// where Component.Output == (WholeMatch, Capture...),
@_disfavoredOverload
public static func buildPartialBlock<R: RegexComponent>(
first r: Component<R>
) -> Regex<Substring>
public static func buildPartialBlock<W, C0, R: RegexComponent>(
first r: Component<R>
) -> Regex<(Substring, C0?)> where R.Output == (W, C0)
public static func buildPartialBlock<W, C0, C1, R: RegexComponent>(
first r: Component<R>
) -> Regex<(Substring, C0?, C1?)> where R.Output == (W, C0, C1)
// The following builder methods implement what would be possible with
// variadic generics (using imaginary syntax) as a single method:
//
// public static func buildPartialBlock<
// AccumulatedWholeMatch, NextWholeMatch,
// AccumulatedCapture..., NextCapture...,
// Accumulated: RegexComponent, Next: RegexComponent
// >(
// accumulated: Accumulated, next: Component<Next>
// ) -> Regex<(Substring, AccumulatedCapture..., NextCapture...)>
// where Accumulated.Output == (AccumulatedWholeMatch, AccumulatedCapture...),
// Next.Output == (NextWholeMatch, NextCapture...)
public static func buildPartialBlock<W0, W1, C0, R0: RegexComponent, R1: RegexComponent>(
accumulated: R0, next: Component<R1>
) -> Regex<(Substring, C0?)> where R0.Output == W0, R1.Output == (W1, C0)
public static func buildPartialBlock<W0, W1, C0, C1, R0: RegexComponent, R1: RegexComponent>(
accumulated: R0, next: Component<R1>
) -> Regex<(Substring, C0?, C1?)> where R0.Output == W0, R1.Output == (W1, C0, C1)
public static func buildPartialBlock<W0, W1, C0, C1, C2, R0: RegexComponent, R1: RegexComponent>(
accumulated: R0, next: Component<R1>
) -> Regex<(Substring, C0?, C1?, C2?)> where R0.Output == W0, R1.Output == (W1, C0, C1, C2)
// ... `O(arity^2)` overloads of `buildPartialBlock(accumulated:next:)`
}
extension AlternationBuilder {
// The following builder methods implement what would be possible with
// variadic generics (using imaginary syntax) as a single method:
//
// public static func buildLimitedAvailability<
// Component, WholeMatch, Capture...
// >(
// _ component: Component
// ) -> Regex<(Substring, Capture?...)>
// where Component.Output == (WholeMatch, Capture...)
@_disfavoredOverload
public static func buildLimitedAvailability<R: RegexComponent>(
_ component: Component<R>
) -> Regex<Substring>
public static func buildLimitedAvailability<W, C0, R: RegexComponent>(
_ component: Component<R>
) -> Regex<(Substring, C0?)>
public static func buildLimitedAvailability<W, C0, C1, R: RegexComponent>(
_ component: Component<R>
) -> Regex<(Substring, C0?, C1?)>
// ... `O(arity)` overloads of `buildLimitedAvailability(_:)`
public static func buildLimitedAvailability<W, C0, C1, C2, C3, C4, C5, C6, C7, C8, C9, R: RegexComponent>(
_ component: Component<R>
) -> Regex<(Substring, C0?, C1?, C2?, C3?, C4?, C5?, C6?, C7?, C8, C9?)> where R.Output == (W, C0, C1, C2, C3, C4, C5, C6, C7, C8, C9)
}
```
### Quantification
Quantifiers are free functions that take a regex or a `@RegexComponentBuilder` closure that produces a regex. The result is a regex whose `Output` type is the same as the argument's, when the lower bound of quantification is greater than `0`; otherwise, it is an `Optional` thereof.
Quantifiers are generic types that can be created from a regex component. Their `Output` type is inferred from initializers. Each of these types corresponds to a quantifier in the textual regex.
| Quantifier in regex builder | Quantifier in textual regex |
|-----------------------------|-----------------------------|
| `OneOrMore(...)` | `...+` |
| `ZeroOrMore(...)` | `...*` |
| `Optionally(...)` | `...?` |
| `Repeat(..., count: n)` | `...{n}` |
| `Repeat(..., n...)` | `...{n,}` |
| `Repeat(..., n...m)` | `...{n,m}` |
```swift
public struct OneOrMore<Output>: RegexComponent {
public var regex: Regex<Output> { get }
}
public struct ZeroOrMore<Output>: RegexComponent {
public var regex: Regex<Output> { get }
}
public struct Optionally<Output>: RegexComponent {
public var regex: Regex<Output> { get }
}
public struct Repeat<Output>: RegexComponent {
public var regex: Regex<Output> { get }
}
```
Like quantifiers in textual regexes, the developer can specify how eager the pattern should be matched against using `QuantificationBehavior`. Static properties in `QuantificationBehavior` are named like adverbs for fluency at a quantifier call site.
```swift
/// Specifies how much to attempt to match when using a quantifier.
public struct QuantificationBehavior {
/// Match as much of the input string as possible, backtracking when
/// necessary.
public static var eagerly: QuantificationBehavior { get }
/// Match as little of the input string as possible, expanding the matched
/// region as necessary to complete a match.
public static var reluctantly: QuantificationBehavior { get }
/// Match as much of the input string as possible, performing no backtracking.
public static var possessively: QuantificationBehavior { get }
}
```
Each quantification behavior corresponds to a quantification behavior in the textual regex.
| Quantifier behavior in regex builder | Quantifier behavior in textual regex |
|--------------------------------------|--------------------------------------|
| `.eagerly` | no suffix |
| `.reluctantly` | suffix `?` |
| `.possessively` | suffix `+` |
`OneOrMore` and count-based `Repeat` are quantifiers that produce a new regex with the original capture types. Their `Output` type is `Substring` followed by the component's capture types. `ZeroOrMore`, `Optionally`, and range-based `Repeat` are quantifiers that produce a new regex with optional capture types. Their `Output` type is `Substring` followed by the component's capture types wrapped in `Optional`.
| Quantifier | Component `Output` | Result `Output` |
|------------------------------------------------------|----------------------------|----------------------------|
| `OneOrMore`<br>`Repeat(..., count: ...)` | `(WholeMatch, Capture...)` | `(Substring, Capture...)` |
| `OneOrMore`<br>`Repeat(..., count: ...)` | `WholeMatch` (non-tuple) | `Substring` |
| `ZeroOrMore`<br>`Optionally`<br>`Repeat(..., n...m)` | `(WholeMatch, Capture...)` | `(Substring, Capture?...)` |
| `ZeroOrMore`<br>`Optionally`<br>`Repeat(..., n...m)` | `WholeMatch` (non-tuple) | `Substring` |
Due to the lack of variadic generics, these functions must be overloaded for every supported capture arity.
```swift
extension OneOrMore {
// The following builder methods implement what would be possible with
// variadic generics (using imaginary syntax) as a single set of methods:
//
// public init<
// Component: RegexComponent, WholeMatch, Capture...
// >(
// _ component: Component,
// _ behavior: QuantificationBehavior = .eagerly
// )
// where Output == (Substring, Capture...)>,
// Component.Output == (WholeMatch, Capture...)
//
// public init<
// Component: RegexComponent, WholeMatch, Capture...
// >(
// _ behavior: QuantificationBehavior = .eagerly,
// @RegexComponentBuilder _ component: () -> Component
// )
// where Output == (Substring, Capture...),
// Component.Output == (WholeMatch, Capture...)
@_disfavoredOverload
public init<Component: RegexComponent>(
_ component: Component,
_ behavior: QuantificationBehavior = .eagerly
) where Output == Substring
@_disfavoredOverload
public init<Component: RegexComponent>(
_ behavior: QuantificationBehavior = .eagerly,
@RegexComponentBuilder _ component: () -> Component
) where Output == Substring
public init<W, C0, Component: RegexComponent>(
_ component: Component,
_ behavior: QuantificationBehavior = .eagerly
) where Output == (Substring, C0), Component.Output == (W, C0)
public init<W, C0, Component: RegexComponent>(
_ behavior: QuantificationBehavior = .eagerly,
@RegexComponentBuilder _ component: () -> Component
) where Output == (Substring, C0), Component.Output == (W, C0)
// ... `O(arity)` overloads
}
extension ZeroOrMore {
// The following builder methods implement what would be possible with
// variadic generics (using imaginary syntax) as a single set of methods:
//
// public init<
// Component: RegexComponent, WholeMatch, Capture...
// >(
// _ component: Component,
// _ behavior: QuantificationBehavior = .eagerly
// )
// where Output == (Substring, Capture?...)>,
// Component.Output == (WholeMatch, Capture...)
//
// public init<
// Component: RegexComponent, WholeMatch, Capture...
// >(
// _ behavior: QuantificationBehavior = .eagerly,
// @RegexComponentBuilder _ component: () -> Component
// )
// where Output == (Substring, Capture?...),
// Component.Output == (WholeMatch, Capture...)
@_disfavoredOverload
public init<Component: RegexComponent>(
_ component: Component,
_ behavior: QuantificationBehavior = .eagerly
) where Output == Substring
@_disfavoredOverload
public init<Component: RegexComponent>(
_ behavior: QuantificationBehavior = .eagerly,
@RegexComponentBuilder _ component: () -> Component
) where Output == Substring
public init<W, C0, Component: RegexComponent>(
_ component: Component,
_ behavior: QuantificationBehavior = .eagerly
) where Output == (Substring, C0?), Component.Output == (W, C0)
public init<W, C0, Component: RegexComponent>(
_ behavior: QuantificationBehavior = .eagerly,
@RegexComponentBuilder _ component: () -> Component
) where Output == (Substring, C0?), Component.Output == (W, C0)
// ... `O(arity)` overloads
}
extension Optionally {
// The following builder methods implement what would be possible with
// variadic generics (using imaginary syntax) as a single set of methods:
//
// public init<
// Component: RegexComponent, WholeMatch, Capture...
// >(
// _ component: Component,
// _ behavior: QuantificationBehavior = .eagerly
// )
// where Output == (Substring, Capture?...),
// Component.Output == (WholeMatch, Capture...)
//
// public init<
// Component: RegexComponent, WholeMatch, Capture...
// >(
// _ behavior: QuantificationBehavior = .eagerly,
// @RegexComponentBuilder _ component: () -> Component
// )
// where Output == (Substring, Capture?...)>,
// Component.Output == (WholeMatch, Capture...)
@_disfavoredOverload
public init<Component: RegexComponent>(
_ component: Component,
_ behavior: QuantificationBehavior = .eagerly
) where Output == Substring
@_disfavoredOverload
public init<Component: RegexComponent>(
_ behavior: QuantificationBehavior = .eagerly,
@RegexComponentBuilder _ component: () -> Component
) where Output == Substring
public init<W, C0, Component: RegexComponent>(
_ component: Component,
_ behavior: QuantificationBehavior = .eagerly
) where Output == (Substring, C0?), Component.Output == (W, C0)
public init<W, C0, Component: RegexComponent>(
_ behavior: QuantificationBehavior = .eagerly,
@RegexComponentBuilder _ component: () -> Component
) where Output == (Substring, C0?), Component.Output == (W, C0)
// ... `O(arity)` overloads
}
extension Repeat {
// The following builder methods implement what would be possible with
// variadic generics (using imaginary syntax) as a single set of methods:
//
// public init<
// Component: RegexComponent, WholeMatch, Capture...
// >(
// _ component: Component,
// count: Int,
// _ behavior: QuantificationBehavior = .eagerly
// )
// where Output == (Substring, Capture...),
// Component.Output == (WholeMatch, Capture...)
//
// public init<
// Component: RegexComponent, WholeMatch, Capture...
// >(
// count: Int,
// _ behavior: QuantificationBehavior = .eagerly,
// @RegexComponentBuilder _ component: () -> Component
// )
// where Output == (Substring, Capture...),
// Component.Output == (WholeMatch, Capture...)
//
// public init<
// Component: RegexComponent, WholeMatch, Capture..., RE: RangeExpression
// >(
// _ component: Component,
// _ expression: RE,
// _ behavior: QuantificationBehavior = .eagerly
// )
// where Output == (Substring, Capture?...),
// Component.Output == (WholeMatch, Capture...)
//
// public init<
// Component: RegexComponent, WholeMatch, Capture..., RE: RangeExpression
// >(
// _ expression: RE,
// _ behavior: QuantificationBehavior = .eagerly,
// @RegexComponentBuilder _ component: () -> Component
// )
// where Output == (Substring, Capture?...),
// Component.Output == (WholeMatch, Capture...)
// Nullary
@_disfavoredOverload
public init<Component: RegexComponent>(
_ component: Component,
count: Int,
_ behavior: QuantificationBehavior = .eagerly
) where Output == Substring, R.Bound == Int
@_disfavoredOverload
public init<Component: RegexComponent>(
count: Int,
_ behavior: QuantificationBehavior = .eagerly,
@RegexComponentBuilder _ component: () -> Component
) where Output == Substring, R.Bound == Int
@_disfavoredOverload
public init<Component: RegexComponent, RE: RangeExpression>(
_ component: Component,
_ expression: RE,
_ behavior: QuantificationBehavior = .eagerly
) where Output == Substring, R.Bound == Int
@_disfavoredOverload
public init<Component: RegexComponent, RE: RangeExpression>(
_ expression: RE,
_ behavior: QuantificationBehavior = .eagerly,
@RegexComponentBuilder _ component: () -> Component
) where Output == Substring, R.Bound == Int
// Unary
public init<W, C0, Component: RegexComponent>(
_ component: Component,
count: Int,
_ behavior: QuantificationBehavior = .eagerly
)
where Output == (Substring, C0),
Component.Output == (Substring, C0),
R.Bound == Int
public init<W, C0, Component: RegexComponent>(
count: Int,
_ behavior: QuantificationBehavior = .eagerly,
@RegexComponentBuilder _ component: () -> Component
)
where Output == (Substring, C0),
Component.Output == (Substring, C0),
R.Bound == Int
public init<W, C0, Component: RegexComponent, RE: RangeExpression>(
_ component: Component,
_ expression: RE,
_ behavior: QuantificationBehavior = .eagerly
)
where Output == (Substring, C0?),
Component.Output == (W, C0),
R.Bound == Int
public init<W, C0, Component: RegexComponent, RE: RangeExpression>(
_ expression: RE,
_ behavior: QuantificationBehavior = .eagerly,
@RegexComponentBuilder _ component: () -> Component
)
where Output == (Substring, C0?),
Component.Output == (W, C0),
R.Bound == Int
// ... `O(arity)` overloads
}
```
### Capture and reference
`Capture` and `TryCapture` produce a new `Regex` by inserting the captured pattern's whole match (`.0`) to the `.1` position of `Output`. When a transform closure is provided, the whole match of the captured content will be transformed to using the closure.
```swift
public struct Capture<Output>: RegexComponent {
public var regex: Regex<Output> { get }
}
public struct TryCapture<Output>: RegexComponent {
public var regex: Regex<Output> { get }
}
```
The difference between `Capture` and `TryCapture` is that `TryCapture` works better with transform closures that can return `nil` or throw, whereas `Capture` relies on the user to handle errors within a transform closure. With `TryCapture`, when the closure returns `nil` or throws, the failure becomes a no-match.
```swift
// Below are `Capture` and `TryCapture` initializer variants on capture arity 0.
// Higher capture arities are omitted for simplicity.
extension Capture {
public init<R: RegexComponent, W>(
_ component: R
) where Output == (Substring, W), R.Output == W
public init<R: RegexComponent, W>(
_ component: R, as reference: Reference<W>
) where Output == (Substring, W), R.Output == W
public init<R: RegexComponent, W, NewCapture>(
_ component: R,
transform: @escaping (Substring) -> NewCapture
) where Output == (Substring, NewCapture), R.Output == W
public init<R: RegexComponent, W, NewCapture>(
_ component: R,
as reference: Reference<NewCapture>,
transform: @escaping (Substring) -> NewCapture
) where Output == (Substring, NewCapture), R.Output == W
public init<R: RegexComponent, W>(
@RegexComponentBuilder _ component: () -> R
) where Output == (Substring, W), R.Output == W
public init<R: RegexComponent, W>(
as reference: Reference<W>,
@RegexComponentBuilder _ component: () -> R
) where Output == (Substring, W), R.Output == W
}
extension TryCapture {
public init<R: RegexComponent, W, NewCapture>(
_ component: R,
transform: @escaping (Substring) throws -> NewCapture
) where Output == (Substring, NewCapture), R.Output == W
public init<R: RegexComponent, W, NewCapture>(
_ component: R,
as reference: Reference<NewCapture>,
transform: @escaping (Substring) throws -> NewCapture
) where Output == (Substring, NewCapture), R.Output == W
public init<R: RegexComponent, W, NewCapture>(
_ component: R,
transform: @escaping (Substring) -> NewCapture?
) where Output == (Substring, NewCapture), R.Output == W
public init<R: RegexComponent, W, NewCapture>(
_ component: R,
as reference: Reference<NewCapture>,
transform: @escaping (Substring) -> NewCapture?
) where Output == (Substring, NewCapture), R.Output == W
public init<R: RegexComponent, W, NewCapture>(
@RegexComponentBuilder _ component: () -> R,
transform: @escaping (Substring) -> NewCapture
) where Output == (Substring, NewCapture), R.Output == W
public init<R: RegexComponent, W, NewCapture>(
as reference: Reference<NewCapture>,
@RegexComponentBuilder _ component: () -> R,
transform: @escaping (Substring) throws -> NewCapture
) where Output == (Substring, NewCapture), R.Output == W
public init<R: RegexComponent, W, NewCapture>(
@RegexComponentBuilder _ component: () -> R,
transform: @escaping (Substring) -> NewCapture?
) where Output == (Substring, NewCapture), R.Output == W
public init<R: RegexComponent, W, NewCapture>(
as reference: Reference<NewCapture>,
@RegexComponentBuilder _ component: () -> R,
transform: @escaping (Substring) -> NewCapture?
) where Output == (Substring, NewCapture), R.Output == W
// ... `O(arity)` overloads
}
```
Example:
```swift
let regex = Regex {
OneOrMore("a")
Capture {
TryCapture("b") { Int($0) }
ZeroOrMore {
TryCapture("c") { Double($0) }
}
Optionally("e")
}
}
```
Variants of `Capture` and `TryCapture` accept a `Reference` argument. References can be used to achieve named captures and named backreferences from textual regexes.
```swift
public struct Reference<Capture>: RegexComponent {
public init(_ captureType: Capture.Type = Capture.self)
public var regex: Regex<Capture>
}
extension Regex.Match {
public subscript<Capture>(_ reference: Reference<Capture>) -> Capture { get }
}
```
When capturing some regex with a reference specified, the reference will refer to the most recently captured content. The reference itself can be used as a regex to match the most recently captured content, or as a name to look up the result of matching.
```swift
let a = Reference(Substring.self)
let b = Reference(Substring.self)
let regex = Regex {
Capture("abc", as: a)
Capture("def", as: b)
a
Capture(b)
}
if let result = input.firstMatch(of: regex) {
print(result[a]) // => "abc"
print(result[b]) // => "def"
}
```
A regex is considered invalid when it contains a use of reference without it ever being captured in the regex. When this occurs in the regex builder DSL, an runtime error will be reported.
### Subpattern
In textual regex, one can refer to a subpattern to avoid duplicating the subpattern, for example:
```
(you|I) say (goodbye|hello); (?1) say (?2)
```
The above regex is equivalent to
```
(you|I) say (goodbye|hello); (you|I) say (goodbye|hello)
```
With regex builder, there is no special API required to reuse existing subpatterns, as a subpattern can be defined modularly using a `let` binding inside or outside a regex builder closure.
```swift
Regex {
let subject = ChoiceOf {
"I"
"you"
}
let object = ChoiceOf {
"goodbye"
"hello"
}
subject
"say"
object
";"
subject
"say"
object
}
```
Sometimes, a textual regex may also use `(?R)` or `(?0)` to recusively evaluate the entire regex. For example, the following textual regex matches "I say you say I say you say hello".
```
(you|I) say (goodbye|hello|(?R))
```
For this, `Regex` offers a special initializer that allows its pattern to recursively reference itself. This is somewhat akin to a fixed-point combinator.
```swift
extension Regex {
public init<R: RegexComponent>(
@RegexComponentBuilder _ content: (Regex<Substring>) -> R
) where R.Output == Match
}
```
With this initializer, the above regex can be expressed as the following using regex builder.
```swift
Regex { wholeSentence in
ChoiceOf {
"I"
"you"
}
"say"
ChoiceOf {
"goodbye"
"hello"
wholeSentence
}
}
```
### Scoping
In textual regexes, atomic groups (`(?>...)`) can be used to define a backtracking scope. That is, when the regex engine exits from the scope successfully, it throws away all backtracking positions from the scope. In regex builder, the `Local` type serves this purpose.
```swift
public struct Local<Output>: RegexComponent {
public var regex: Regex<Output>
// The following builder methods implement what would be possible with
// variadic generics (using imaginary syntax) as a single set of methods:
//
// public init<WholeMatch, Capture..., Component: RegexComponent>(
// @RegexComponentBuilder _ component: () -> Component
// ) where Output == (Substring, Capture...), Component.Output == (WholeMatch, Capture...)
@_disfavoredOverload
public init<Component: RegexComponent>(
@RegexComponentBuilder _ component: () -> Component
) where Output == Substring
public init<W, C0, Component: RegexComponent>(
@RegexComponentBuilder _ component: () -> Component
) where Output == (Substring, C0), Component.Output == (W, C0)
public init<W, C0, C1, Component: RegexComponent>(
@RegexComponentBuilder _ component: () -> Component
) where Output == (Substring, C0, C1), Component.Output == (W, C0, C1)
// ... `O(arity)` overloads
}
```
For example, the following regex matches string `abcc` but not `abc`.
```swift
Regex {
"a"
Local {
ChoiceOf {
"bc"
"b"
}
}
"c"
}
```
## Source compatibility
Regex builder will be shipped in a new module named `RegexBuilder`, and thus will not affect the source compatibility of the existing code.
## Effect on ABI stability
The proposed feature does not change the ABI of existing features.
## Effect on API resilience
The proposed feature relies heavily upon overloads of `buildBlock` and `buildPartialBlock(accumulated:next:)` to work for different capture arities. In the fullness of time, we are hoping for variadic generics to supercede existing overloads. Such a change should not involve ABI-breaking modifications as it is merely a change of overload resolution.
## Alternatives considered
### Operators for quantification and alternation
While `ChoiceOf` and quantifier types provide a general way of creating alternations and quantifications, we recognize that some synctactic sugar can be useful for creating one-liners like in textual regexes, e.g. infix operator `|`, postfix operator `*`, etc.
```swift
// The following functions implement what would be possible with variadic
// generics (using imaginary syntax) as a single function:
//
// public func | <
// R0: RegexComponent, R1: RegexComponent,
// WholeMatch0, WholeMatch1,
// Capture0..., Capture1...
// >(
// _ r0: RegexComponent,
// _ r1: RegexComponent
// ) -> Regex<(Substring, Capture0?..., Capture1?...)>
// where R0.Output == (WholeMatch0, Capture0...),
// R1.Output == (WholeMatch1, Capture1...)
@_disfavoredOverload
public func | <R0, R1>(lhs: R0, rhs: R1) -> Regex<Substring> where R0: RegexComponent, R1: RegexComponent {
public func | <R0, R1, W1, C0>(lhs: R0, rhs: R1) -> Regex<(Substring, C0?)> where R0: RegexComponent, R1: RegexComponent, R1.Output == (W1, C0)
public func | <R0, R1, W1, C0, C1>(lhs: R0, rhs: R1) -> Regex<(Substring, C0?, C1?)> where R0: RegexComponent, R1: RegexComponent, R1.Output == (W1, C0, C1)
// ... `O(arity^2)` overloads.
```
However, like `RegexComponentBuilder.buildPartialBlock(accumulated:next:)`, operators such as `|`, `+`, `*`, `.?` require a large number of overloads to work with regexes of every capture arity, compounded by the fact that operator type checking is prone to performance issues in Swift. Here is a list of
| Opreator | Meaning | Required number of overloads |
|---------------|---------------------------|------------------------------|
| Infix `\|` | Choice of two | `O(arity^2)` |
| Postfix `*` | Zero or more eagerly | `O(arity)` |
| Postfix `*?` | Zero or more reluctantly | `O(arity)` |
| Postfix `*+` | Zero or more possessively | `O(arity)` |
| Postfix `+` | One or more eagerly | `O(arity)` |
| Postfix `+?` | One or more reluctantly | `O(arity)` |
| Postfix `++` | One or more possessively | `O(arity)` |
| Postfix `.?` | Optionally eagerly | `O(arity)` |
| Postfix `.??` | Optionally reluctantly | `O(arity)` |
| Postfix `.?+` | Optionally possessively | `O(arity)` |
When variadic generics are supported in the future, we may be able to define one function per operator and reduce type checking burdens.
### Postfix `capture` and `tryCapture` methods
An earlier iteration of regex builder declared `capture` and `tryCapture` as methods on `RegexComponent`, meaning that you can append `.capture(...)` to any subpattern within a regex to capture it. For example:
```swift
Regex {
OneOrMore {
r0.capture()
r1
}.capture()
} // => Regex<(Substring, Substring, Substring)>
```
However, there are two shortcomings of this design:
1. When a subpattern to be captured contains multiple components, the developer has to explicitly group them using a `Regex { ... }` block.
```swift
let emailPattern = Regex {
let word = OneOrMore(.word)
Regex { // <= Had to explicitly group multiple components
ZeroOrMore {
word
"."
}
word
}.capture()
"@"
Regex {
word
OneOrMore {
"."
word
}
}.capture()
} // => Regex<(Substring, Substring, Substring)>
```
2. When there are nested captures, it is harder to number the captures visually because the order `capture()` appears is flipped in the postfix (method) notation.
```swift
let emailSuffixPattern = Regex {
"@"
Regex {
word
OneOrMore {
"."
word.capture() // top-level domain (.0)
}
}.capture() // full domain (.1)
} // => Regex<(Substring, Substring, Substring)>
//
// full domain ^~~~~~~~~
// top-level domain ^~~~~~~~~
```
In comparison, prefix notation (`Capture` and `TryCapture` as a types) makes it easier to visually capture captures as you can number captures in the order they appear from top to bottom. This is consistent with textual regexes where capturing groups are numbered by the left parenthesis of the group from left to right.
```swift
let emailSuffixPattern = Regex {
Capture { // full domain (.0)
word
OneOrMore {
"."
Capture(word) // top-level domain (.1)
}
}
} // => Regex<(Substring, Substring, Substring)>
//
// full domain ^~~~~~~~~
// top-level domain ^~~~~~~~~
```
### Unify quantifiers under `Repeat`
Since `Repeat` is the most general version of quantifiers, one could argue for all quantifiers to be unified under the type `Repeat`, for example:
```swift
Repeat(oneOrMore: r)
Repeat(zeroOrMore: r)
Repeat(optionally: r)
```
However, given that one-or-more (`+`), zero-or-more (`*`) and optional (`?`) are the most common quantifiers in textual regexes, we believe that these quantifiers deserve their own type and should be written as a single word instead of two. This can also reduce visual clutter when the quantification is used in multiple places of a regex.
### Free functions instead of types
One could argue that type such as `OneOrMore<Output>` could be defined as a top-level function that returns `Regex`. While it is entirely possible to do so, it would lose the name scoping benefits of a type and pollute the top-level namespace with `O(arity^2)` overloads of quantifiers, `capture`, `tryCapture`, etc. This could be detrimental to the usefulness of code completion.
Another reason to use types instead of free functions is consistency with existing result-builder-based DSLs such as SwiftUI.
### Support `buildOptional` and `buildEither`
To support `if` statements, an earlier iteration of this proposal defined `buildEither(first:)`, `buildEither(second:)` and `buildOptional(_:)` as the following:
```swift
extension RegexComponentBuilder {
public static func buildEither<
Component, WholeMatch, Capture...
>(
first component: Component
) -> Regex<(Substring, Capture...)>
where Component.Output == (WholeMatch, Capture...)
public static func buildEither<
Component, WholeMatch, Capture...
>(
second component: Component
) -> Regex<(Substring, Capture...)>
where Component.Output == (WholeMatch, Capture...)
public static func buildOptional<
Component, WholeMatch, Capture...
>(
_ component: Component?
) where Component.Output == (WholeMatch, Capture...)
}
```
However, multiple-branch control flow statements (e.g. `if`-`else` and `switch`) would need to be required to produce either the same regex type, which is limiting, or an "either-like" type, which can be difficult to work with when nested. Unlike `ChoiceOf`, producing a tuple of optionals is not an option, because the branch taken would be decided when the builder closure is executed, and it would cause capture numbering to be inconsistent with conventional regex.
Moreover, result builder conditionals does not work the same way as regex conditionals. In regex conditionals, the conditions are themselves regexes and are evaluated by the regex engine during matching, whereas result builder conditionals are evaluated as part of the builder closure. We hope that a future result builder feature will support "lifting" control flow conditions into the DSL domain, e.g. supporting `Regex<Bool>` as a condition.
### Flatten optionals
With the proposed design, `ChoiceOf` with `AlternationBuilder` wraps every component's capture type with an `Optional`. This means that any `ChoiceOf` with optional-capturing components would lead to a doubly-nested optional captures. This could make the result of matching harder to use.
```swift
ChoiceOf {
OneOrMore(Capture(.digit)) // Output == (Substring, Substring)
Optionally {
ZeroOrMore(Capture(.word)) // Output == (Substring, Substring?)
"a"
} // Output == (Substring, Substring??)
} // Output == (Substring, Substring?, Substring???)
```
One way to improve this could be overloading quantifier initializers (e.g. `ZeroOrMore.init(_:)`) and `AlternationBuilder.buildPartialBlock` to flatten any optionals upon composition. However, this would be non-trivial. Quantifier initializers would need to be overloaded `O(2^arity)` times to account for all possible positions of `Optional` that may appear in the `Output` tuple. Even worse, `AlternationBuilder.buildPartialBlock` would need to be overloaded `O(arity!)` times to account for all possible combinations of two `Output` tuples with all possible positions of `Optional` that may appear in one of the `Output` tuples.
### Structured rather than flat captures
We propose inferring capture types in such a way as to align with the traditional numbering of backreferences. This is because much of the motivation behind providing regex in Swift is their familiarity.
If we decided to deprioritize this motivation, there are opportunities to infer safer, more ergonomic, and arguably more intuitive types for captures. For example, to be consistent with traditional regex backreferences quantifications of multiple or nested captures had to produce parallel arrays rather than an array of tuples.
```swift
OneOrMore {
Capture {
OneOrMore(.hexDigit)
}
".."
Capture {
OneOrMore(.hexDigit)
}
}
// Flat capture types:
// => `Output == (Substring, Substring, Substring)>`
// Structured capture types:
// => `Output == (Substring, (Substring, Substring))`
```
Similarly, an alternation of multiple or nested captures could produce a structured alternation type (or an anonymous sum type) rather than flat optionals.
This is cool, but it adds extra complexity to regex builder and it isn't as clear because the generic type no longer aligns with the traditional regex backreference numbering. We think the consistency of the flat capture types trumps the added safety and ergonomics of the structured capture types.
[Declarative String Processing]: https://github.com/apple/swift-experimental-string-processing/blob/main/Documentation/DeclarativeStringProcessing.md
[Strongly Typed Regex Captures]: https://github.com/apple/swift-experimental-string-processing/blob/main/Documentation/Evolution/StronglyTypedCaptures.md
[Regex Syntax]: https://github.com/apple/swift-experimental-string-processing/blob/main/Documentation/Evolution/RegexSyntax.md
|