New: Add no-useless-backreference rule (fixes #12673) (#12690)

* New: Add no-useless-backreference rule (fixes #12673)

* Improve opening section in the docs, fix typo in a comment

* Reword the opening paragraph

docs/rules/no-useless-backreference.md

@@ -0,0 +1,130 @@
+# Disallow useless backreferences in regular expressions (no-useless-backreference)
+
+In JavaScript regular expressions, it's syntactically valid to define a backreference to a group that belongs to another alternative part of the pattern, a backreference to a group that appears after the backreference, a backreference to a group that contains that backreference, or a backreference to a group that is inside a negative lookaround. However, by the specification, in any of these cases the backreference always ends up matching only zero-length (the empty string), regardless of the context in which the backreference and the group appear.
+
+Backreferences that always successfully match zero-length and cannot match anything else are useless. They are basically ignored and can be removed without changing the behavior of the regular expression.
+
+```js
+var regex = /^(?:(a)|\1b)$/;
+
+regex.test("a"); // true
+regex.test("b"); // true!
+regex.test("ab"); // false
+
+var equivalentRegex = /^(?:(a)|b)$/;
+
+equivalentRegex.test("a"); // true
+equivalentRegex.test("b"); // true
+equivalentRegex.test("ab"); // false
+```
+
+Useless backreference is a possible error in the code. It usually indicates that the regular expression does not work as intended.
+
+## Rule Details
+
+This rule aims to detect and disallow the following backreferences in regular expression:
+
+* Backreference to a group that is in another alternative, e.g., `/(a)|\1b/`. In such constructed regular expression, the backreference is expected to match what's been captured in, at that point, a non-participating group.
+* Backreference to a group that appears later in the pattern, e.g., `/\1(a)/`. The group hasn't captured anything yet, and ECMAScript doesn't support forward references. Inside lookbehinds, which match backward, the opposite applies and this rule disallows backreference to a group that appears before in the same lookbehind, e.g., `/(?<=(a)\1)b/`.
+* Backreference to a group from within the same group, e.g., `/(\1)/`. Similar to the previous, the group hasn't captured anything yet, and ECMAScript doesn't support nested references.
+* Backreference to a group that is in a negative lookaround, if the backreference isn't in the same negative lookaround, e.g., `/a(?!(b)).\1/`. A negative lookaround (lookahead or lookbehind) succeeds only if its pattern cannot match, meaning that the group has failed.
+
+By the ECMAScript specification, all backreferences listed above are valid, always succeed to match zero-length, and cannot match anything else. Consequently, they don't produce parsing or runtime errors, but also don't affect the behavior of their regular expressions. They are syntactically valid but useless.
+
+This might be surprising to developers coming from other languages where some of these backreferences can be used in a meaningful way.
+
+```js
+// in some other languages, this pattern would successfully match "aab"
+
+/^(?:(a)(?=a)|\1b)+$/.test("aab"); // false
+```
+
+Examples of **incorrect** code for this rule:
+
+```js
+/*eslint no-useless-backreference: "error"*/
+
+/^(?:(a)|\1b)$/; // reference to (a) into another alternative
+
+/^(?:(a)|b(?:c|\1))$/; // reference to (a) into another alternative
+
+/^(?:a|b(?:(c)|\1))$/; // reference to (c) into another alternative
+
+/\1(a)/; // forward reference to (a)
+
+RegExp('(a)\\2(b)'); // forward reference to (b)
+
+/(?:a)(b)\2(c)/; // forward reference to (c)
+
+/\k<foo>(?<foo>a)/; // forward reference to (?<foo>a)
+
+/(?<=(a)\1)b/; // backward reference to (a) from within the same lookbehind
+
+/(?<!(a)\1)b/; // backward reference to (a) from within the same lookbehind
+
+new RegExp('(\\1)'); // nested reference to (\1)
+
+/^((a)\1)$/; // nested reference to ((a)\1)
+
+/a(?<foo>(.)b\1)/; // nested reference to (?<foo>(.)b\1)
+
+/a(?!(b)).\1/; // reference to (b) into a negative lookahead
+
+/(?<!(a))b\1/; // reference to (a) into a negative lookbehind
+```
+
+Examples of **correct** code for this rule:
+
+```js
+/*eslint no-useless-backreference: "error"*/
+
+/^(?:(a)|(b)\2)$/; // reference to (b)
+
+/(a)\1/; // reference to (a)
+
+RegExp('(a)\\1(b)'); // reference to (a)
+
+/(a)(b)\2(c)/; // reference to (b)
+
+/(?<foo>a)\k<foo>/; // reference to (?<foo>a)
+
+/(?<=\1(a))b/; // reference to (a), correctly before the group as they're in the same lookbehind
+
+/(?<=(a))b\1/; // reference to (a), correctly after the group as the backreference isn't in the lookbehind
+
+new RegExp('(.)\\1'); // reference to (.)
+
+/^(?:(a)\1)$/; // reference to (a)
+
+/^((a)\2)$/; // reference to (a)
+
+/a(?<foo>(.)b\2)/; // reference to (.)
+
+/a(?!(b|c)\1)./; // reference to (b|c), correct as it's from within the same negative lookahead
+
+/(?<!\1(a))b/; // reference to (a), correct as it's from within the same negative lookbehind
+```
+
+Please note that this rule does not aim to detect and disallow a potentially erroneous use of backreference syntax in regular expressions, like the use in character classes or an attempt to reference a group that doesn't exist. Depending on the context, a `\1`...`\9` sequence that is not a syntactically valid backreference may produce syntax error, or be parsed as something else (e.g., as a legacy octal escape sequence).
+
+Examples of additional **correct** code for this rule:
+
+```js
+/*eslint no-useless-backreference: "error"*/
+
+// comments describe behavior in a browser
+
+/^[\1](a)$/.test("\x01a"); // true. In a character class, \1 is treated as an octal escape sequence.
+/^\1$/.test("\x01"); // true. Since the group 1 doesn't exist, \1 is treated as an octal escape sequence.
+/^(a)\1\2$/.test("aa\x02"); // true. In this case, \1 is a backreference, \2 is an octal escape sequence.
+```
+
+## Further Reading
+
+* [MDN: Regular Expressions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions)
+
+## Related Rules
+
+* [no-control-regex](no-control-regex.md)
+* [no-empty-character-class](no-empty-character-class.md)
+* [no-invalid-regexp](no-invalid-regexp.md)

lib/rules/index.js

@@ -216,6 +216,7 @@ module.exports = new LazyLoadingRuleMap(Object.entries({
     "no-unused-labels": () => require("./no-unused-labels"),
     "no-unused-vars": () => require("./no-unused-vars"),
     "no-use-before-define": () => require("./no-use-before-define"),
+    "no-useless-backreference": () => require("./no-useless-backreference"),
     "no-useless-call": () => require("./no-useless-call"),
     "no-useless-catch": () => require("./no-useless-catch"),
     "no-useless-computed-key": () => require("./no-useless-computed-key"),

lib/rules/no-useless-backreference.js

@@ -0,0 +1,193 @@
+/**
+ * @fileoverview Rule to disallow useless backreferences in regular expressions
+ * @author Milos Djermanovic
+ */
+
+"use strict";
+
+//------------------------------------------------------------------------------
+// Requirements
+//------------------------------------------------------------------------------
+
+const { CALL, CONSTRUCT, ReferenceTracker, getStringIfConstant } = require("eslint-utils");
+const { RegExpParser, visitRegExpAST } = require("regexpp");
+const lodash = require("lodash");
+
+//------------------------------------------------------------------------------
+// Helpers
+//------------------------------------------------------------------------------
+
+const parser = new RegExpParser();
+
+/**
+ * Finds the path from the given `regexpp` AST node to the root node.
+ * @param {regexpp.Node} node Node.
+ * @returns {regexpp.Node[]} Array that starts with the given node and ends with the root node.
+ */
+function getPathToRoot(node) {
+    const path = [];
+    let current = node;
+
+    do {
+        path.push(current);
+        current = current.parent;
+    } while (current);
+
+    return path;
+}
+
+/**
+ * Determines whether the given `regexpp` AST node is a lookaround node.
+ * @param {regexpp.Node} node Node.
+ * @returns {boolean} `true` if it is a lookaround node.
+ */
+function isLookaround(node) {
+    return node.type === "Assertion" &&
+        (node.kind === "lookahead" || node.kind === "lookbehind");
+}
+
+/**
+ * Determines whether the given `regexpp` AST node is a negative lookaround node.
+ * @param {regexpp.Node} node Node.
+ * @returns {boolean} `true` if it is a negative lookaround node.
+ */
+function isNegativeLookaround(node) {
+    return isLookaround(node) && node.negate;
+}
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+module.exports = {
+    meta: {
+        type: "problem",
+
+        docs: {
+            description: "disallow useless backreferences in regular expressions",
+            category: "Possible Errors",
+            recommended: false,
+            url: "https://eslint.org/docs/rules/no-useless-backreference"
+        },
+
+        schema: [],
+
+        messages: {
+            nested: "Backreference '{{ bref }}' will be ignored. It references group '{{ group }}' from within that group.",
+            forward: "Backreference '{{ bref }}' will be ignored. It references group '{{ group }}' which appears later in the pattern.",
+            backward: "Backreference '{{ bref }}' will be ignored. It references group '{{ group }}' which appears before in the same lookbehind.",
+            disjunctive: "Backreference '{{ bref }}' will be ignored. It references group '{{ group }}' which is in another alternative.",
+            intoNegativeLookaround: "Backreference '{{ bref }}' will be ignored. It references group '{{ group }}' which is in a negative lookaround."
+        }
+    },
+
+    create(context) {
+
+        /**
+         * Checks and reports useless backreferences in the given regular expression.
+         * @param {ASTNode} node Node that represents regular expression. A regex literal or RegExp constructor call.
+         * @param {string} pattern Regular expression pattern.
+         * @param {string} flags Regular expression flags.
+         * @returns {void}
+         */
+        function checkRegex(node, pattern, flags) {
+            let regExpAST;
+
+            try {
+                regExpAST = parser.parsePattern(pattern, 0, pattern.length, flags.includes("u"));
+            } catch (e) {
+
+                // Ignore regular expressions with syntax errors
+                return;
+            }
+
+            visitRegExpAST(regExpAST, {
+                onBackreferenceEnter(bref) {
+                    const group = bref.resolved,
+                        brefPath = getPathToRoot(bref),
+                        groupPath = getPathToRoot(group);
+                    let messageId = null;
+
+                    if (brefPath.includes(group)) {
+
+                        // group is bref's ancestor => bref is nested ('nested reference') => group hasn't matched yet when bref starts to match.
+                        messageId = "nested";
+                    } else {
+
+                        // Start from the root to find the lowest common ancestor.
+                        let i = brefPath.length - 1,
+                            j = groupPath.length - 1;
+
+                        do {
+                            i--;
+                            j--;
+                        } while (brefPath[i] === groupPath[j]);
+
+                        const indexOfLowestCommonAncestor = j + 1,
+                            groupCut = groupPath.slice(0, indexOfLowestCommonAncestor),
+                            commonPath = groupPath.slice(indexOfLowestCommonAncestor),
+                            lowestCommonLookaround = commonPath.find(isLookaround),
+                            isMatchingBackward = lowestCommonLookaround && lowestCommonLookaround.kind === "lookbehind";
+
+                        if (!isMatchingBackward && bref.end <= group.start) {
+
+                            // bref is left, group is right ('forward reference') => group hasn't matched yet when bref starts to match.
+                            messageId = "forward";
+                        } else if (isMatchingBackward && group.end <= bref.start) {
+
+                            // the opposite of the previous when the regex is matching backward in a lookbehind context.
+                            messageId = "backward";
+                        } else if (lodash.last(groupCut).type === "Alternative") {
+
+                            // group's and bref's ancestor nodes below the lowest common ancestor are sibling alternatives => they're disjunctive.
+                            messageId = "disjunctive";
+                        } else if (groupCut.some(isNegativeLookaround)) {
+
+                            // group is in a negative lookaround which isn't bref's ancestor => group has already failed when bref starts to match.
+                            messageId = "intoNegativeLookaround";
+                        }
+                    }
+
+                    if (messageId) {
+                        context.report({
+                            node,
+                            messageId,
+                            data: {
+                                bref: bref.raw,
+                                group: group.raw
+                            }
+                        });
+                    }
+                }
+            });
+        }
+
+        return {
+            "Literal[regex]"(node) {
+                const { pattern, flags } = node.regex;
+
+                checkRegex(node, pattern, flags);
+            },
+            Program() {
+                const scope = context.getScope(),
+                    tracker = new ReferenceTracker(scope),
+                    traceMap = {
+                        RegExp: {
+                            [CALL]: true,
+                            [CONSTRUCT]: true
+                        }
+                    };
+
+                for (const { node } of tracker.iterateGlobalReferences(traceMap)) {
+                    const [patternNode, flagsNode] = node.arguments,
+                        pattern = getStringIfConstant(patternNode, scope),
+                        flags = getStringIfConstant(flagsNode, scope);
+
+                    if (typeof pattern === "string") {
+                        checkRegex(node, pattern, flags || "");
+                    }
+                }
+            }
+        };
+    }
+};