Fix #27802 - show a warning for a regex that contains a backreference. Ripgrep interprets this as an octal escape sequence, instead of a backreference.

src/vs/base/common/strings.ts

@@ -231,6 +231,10 @@ export function regExpLeadsToEndlessLoop(regexp: RegExp): boolean {
 	return (match && <any>regexp.lastIndex === 0);
 }
 
+export function regExpContainsBackreference(regexpValue: string): boolean {
+	return !!regexpValue.match(/([^\\]|^)(\\\\)*\\\d+/);
+}
+
 /**
  * The normalize() method returns the Unicode Normalization Form of a given string. The form will be
  * the Normalization Form Canonical Composition.

src/vs/base/test/common/strings.test.ts

@@ -312,6 +312,22 @@ suite('Strings', () => {
 		assert(regExpWithFlags.multiline);
 	});
 
+	test('regExpContainsBackreference', () => {
+		assert(strings.regExpContainsBackreference('foo \\5 bar'));
+		assert(strings.regExpContainsBackreference('\\2'));
+		assert(strings.regExpContainsBackreference('(\\d)(\\n)(\\1)'));
+		assert(strings.regExpContainsBackreference('(A).*?\\1'));
+		assert(strings.regExpContainsBackreference('\\\\\\1'));
+		assert(strings.regExpContainsBackreference('foo \\\\\\1'));
+
+		assert(!strings.regExpContainsBackreference(''));
+		assert(!strings.regExpContainsBackreference('\\\\1'));
+		assert(!strings.regExpContainsBackreference('foo \\\\1'));
+		assert(!strings.regExpContainsBackreference('(A).*?\\\\1'));
+		assert(!strings.regExpContainsBackreference('foo \\d1 bar'));
+		assert(!strings.regExpContainsBackreference('123'));
+	});
+
 	test('getLeadingWhitespace', () => {
 		assert.equal(strings.getLeadingWhitespace('  foo'), '  ');
 		assert.equal(strings.getLeadingWhitespace('  foo', 2), '');

src/vs/workbench/parts/search/browser/searchWidget.ts

@@ -11,7 +11,7 @@ import { Widget } from 'vs/base/browser/ui/widget';
 import { Action } from 'vs/base/common/actions';
 import { ActionBar } from 'vs/base/browser/ui/actionbar/actionbar';
 import { FindInput, IFindInputOptions } from 'vs/base/browser/ui/findinput/findInput';
-import { InputBox } from 'vs/base/browser/ui/inputbox/inputBox';
+import { InputBox, IMessage } from 'vs/base/browser/ui/inputbox/inputBox';
 import { Button } from 'vs/base/browser/ui/button/button';
 import { IKeyboardEvent } from 'vs/base/browser/keyboardEvent';
 import { KeybindingsRegistry } from 'vs/platform/keybinding/common/keybindingsRegistry';
@@ -220,7 +220,7 @@ export class SearchWidget extends Widget {
 	private renderSearchInput(parent: HTMLElement, options: ISearchWidgetOptions): void {
 		let inputOptions: IFindInputOptions = {
 			label: nls.localize('label.Search', 'Search: Type Search Term and press Enter to search or Escape to cancel'),
-			validation: (value: string) => this.validatSearchInput(value),
+			validation: (value: string) => this.validateSearchInput(value),
 			placeholder: nls.localize('search.placeHolder', "Search"),
 			appendCaseSensitiveLabel: appendKeyBindingLabel('', this.keyBindingService2.lookupKeybinding(Constants.ToggleCaseSensitiveActionId), this.keyBindingService2),
 			appendWholeWordsLabel: appendKeyBindingLabel('', this.keyBindingService2.lookupKeybinding(Constants.ToggleWholeWordActionId), this.keyBindingService2),
@@ -309,7 +309,7 @@ export class SearchWidget extends Widget {
 		}
 	}
 
-	private validatSearchInput(value: string): any {
+	private validateSearchInput(value: string): IMessage {
 		if (value.length === 0) {
 			return null;
 		}
@@ -325,6 +325,12 @@ export class SearchWidget extends Widget {
 		if (strings.regExpLeadsToEndlessLoop(regExp)) {
 			return { content: nls.localize('regexp.validationFailure', "Expression matches everything") };
 		}
+
+		if (strings.regExpContainsBackreference(value)) {
+			return { content: nls.localize('regexp.backreferenceValidationFailure', "Backreferences are not supported") };
+		}
+
+		return null;
 	}
 
 	private onSearchInputChanged(): void {