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Viewing file:     no-useless-backreference.js (9.5 KB)      -rw-r--r--

/**
 * @fileoverview Rule to disallow useless backreferences in regular expressions
 * @author Milos Djermanovic
 */

"use strict";

//------------------------------------------------------------------------------
// Requirements
//------------------------------------------------------------------------------

const { CALL, CONSTRUCT, ReferenceTracker, getStringIfConstant } = require("@eslint-community/eslint-utils");
const { RegExpParser, visitRegExpAST } = require("@eslint-community/regexpp");

//------------------------------------------------------------------------------
// 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
//------------------------------------------------------------------------------

/** @type {import('../shared/types').Rule} */
module.exports = {
    meta: {
        type: "problem",

        docs: {
            description: "Disallow useless backreferences in regular expressions",
            recommended: true,
            url: "https://eslint.org/docs/latest/rules/no-useless-backreference"
        },

        schema: [],

        messages: {
            nested: "Backreference '{{ bref }}' will be ignored. It references group '{{ group }}'{{ otherGroups }} from within that group.",
            forward: "Backreference '{{ bref }}' will be ignored. It references group '{{ group }}'{{ otherGroups }} which appears later in the pattern.",
            backward: "Backreference '{{ bref }}' will be ignored. It references group '{{ group }}'{{ otherGroups }} which appears before in the same lookbehind.",
            disjunctive: "Backreference '{{ bref }}' will be ignored. It references group '{{ group }}'{{ otherGroups }} which is in another alternative.",
            intoNegativeLookaround: "Backreference '{{ bref }}' will be ignored. It references group '{{ group }}'{{ otherGroups }} which is in a negative lookaround."
        }
    },

    create(context) {

        const sourceCode = context.sourceCode;

        /**
         * 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, { unicode: flags.includes("u"), unicodeSets: flags.includes("v") });
            } catch {

                // Ignore regular expressions with syntax errors
                return;
            }

            visitRegExpAST(regExpAST, {
                onBackreferenceEnter(bref) {
                    const groups = [bref.resolved].flat(),
                        brefPath = getPathToRoot(bref);

                    const problems = groups.map(group => {
                        const groupPath = getPathToRoot(group);

                        if (brefPath.includes(group)) {

                            // group is bref's ancestor => bref is nested ('nested reference') => group hasn't matched yet when bref starts to match.
                            return {
                                messageId: "nested",
                                group
                            };
                        }


                        // 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 (groupCut.at(-1).type === "Alternative") {

                            // group's and bref's ancestor nodes below the lowest common ancestor are sibling alternatives => they're disjunctive.
                            return {
                                messageId: "disjunctive",
                                group
                            };
                        }
                        if (!isMatchingBackward && bref.end <= group.start) {

                            // bref is left, group is right ('forward reference') => group hasn't matched yet when bref starts to match.
                            return {
                                messageId: "forward",
                                group
                            };
                        }
                        if (isMatchingBackward && group.end <= bref.start) {

                            // the opposite of the previous when the regex is matching backward in a lookbehind context.
                            return {
                                messageId: "backward",
                                group
                            };
                        }
                        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.
                            return {
                                messageId: "intoNegativeLookaround",
                                group
                            };
                        }

                        return null;
                    });

                    if (problems.length === 0 || problems.some(problem => !problem)) {

                        // If there are no problems or no problems with any group then do not report it.
                        return;
                    }

                    let problemsToReport;

                    // Gets problems that appear in the same disjunction.
                    const problemsInSameDisjunction = problems.filter(problem => problem.messageId !== "disjunctive");

                    if (problemsInSameDisjunction.length) {

                        // Only report problems that appear in the same disjunction.
                        problemsToReport = problemsInSameDisjunction;
                    } else {

                        // If all groups appear in different disjunctions, report it.
                        problemsToReport = problems;
                    }

                    const [{ messageId, group }, ...other] = problemsToReport;
                    let otherGroups = "";

                    if (other.length === 1) {
                        otherGroups = " and another group";
                    } else if (other.length > 1) {
                        otherGroups = ` and other ${other.length} groups`;
                    }
                    context.report({
                        node,
                        messageId,
                        data: {
                            bref: bref.raw,
                            group: group.raw,
                            otherGroups
                        }
                    });
                }
            });
        }

        return {
            "Literal[regex]"(node) {
                const { pattern, flags } = node.regex;

                checkRegex(node, pattern, flags);
            },
            Program(node) {
                const scope = sourceCode.getScope(node),
                    tracker = new ReferenceTracker(scope),
                    traceMap = {
                        RegExp: {
                            [CALL]: true,
                            [CONSTRUCT]: true
                        }
                    };

                for (const { node: refNode } of tracker.iterateGlobalReferences(traceMap)) {
                    const [patternNode, flagsNode] = refNode.arguments,
                        pattern = getStringIfConstant(patternNode, scope),
                        flags = getStringIfConstant(flagsNode, scope);

                    if (typeof pattern === "string") {
                        checkRegex(refNode, pattern, flags || "");
                    }
                }
            }
        };
    }
};