Improved prefixes of empty backreferences

Author: RunDevelopment

src/longest-prefix.ts

@@ -1,6 +1,12 @@
 import { CharSet } from "refa";
 import { Alternative, CapturingGroup, Element, Group, Quantifier } from "regexpp/ast";
-import { isPotentiallyZeroLength, isStrictBackreference, isZeroLength, MatchingDirection } from "./basic";
+import {
+	isEmptyBackreference,
+	isPotentiallyZeroLength,
+	isStrictBackreference,
+	isZeroLength,
+	MatchingDirection,
+} from "./basic";
 import { CacheInstance } from "./cache";
 import { ReadonlyFlags } from "./flags";
 import {
@@ -173,6 +179,9 @@ function getElementPrefix(
 			return getQuantifierPrefix(element, direction, options, flags);
 
 		case "Backreference": {
+			if (isEmptyBackreference(element)) {
+				return EMPTY_COMPLETE;
+			}
 			if (isStrictBackreference(element)) {
 				const inner = getElementPrefix(element.resolved, direction, { ...options, includeAfter: false }, flags);
 				return inner;

tests/__snapshots__/longest-prefix.ts.snap

@@ -14,6 +14,20 @@ Object {
 }
 `;
 
+exports[`getLongestPrefix /(?<this>\\1\\2b)c(a)/ ltr 1`] = `
+Object {
+  "{\\"includeAfter\\":false,\\"looseGroups\\":false}": /b/,
+  "{\\"includeAfter\\":true,\\"looseGroups\\":false}": /bc/,
+}
+`;
+
+exports[`getLongestPrefix /(?<this>\\1\\2b)c(a)/ rtl 1`] = `
+Object {
+  "{\\"includeAfter\\":false,\\"looseGroups\\":false}": /b/,
+  "{\\"includeAfter\\":true,\\"looseGroups\\":false}": /b\\[\\^\\]/,
+}
+`;
+
 exports[`getLongestPrefix /(?<this>A(?!a)(?:(?!B)|(?!C)))\\w/ ltr 1`] = `
 Object {
   "{\\"includeAfter\\":false,\\"looseGroups\\":false}": /A/,

tests/longest-prefix.ts

@@ -52,6 +52,7 @@ describe(RAA.getLongestPrefix.name, function () {
 		/(a)b(?<this>\1)c/,
 		/(ab?)b(?<this>\1)c/,
 		/(?:(a)|f)b(?<this>\1)c/,
+		/(?<this>\1\2b)c(a)/,
 	]);
 
 	function test(regexes: RegExp[]): void {