tools: Add no useless regex char class rule

Eslint Rule:
Disallow useless escape in regex character class
with optional override characters option and auto
fixable with eslint --fix option.

Usage:
no-useless-regex-char-class-escape: [2, { override: ['[', ']'] }]

PR-URL: #9591
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>

.eslintrc

@@ -122,6 +122,7 @@ rules:
   align-multiline-assignment: 2
   assert-fail-single-argument: 2
   new-with-error: [2, Error, RangeError, TypeError, SyntaxError, ReferenceError]
+  no-useless-regex-char-class-escape: [2, { override: ['[', ']'] }]
 
 # Global scoped method and vars
 globals:

tools/eslint-rules/no-useless-regex-char-class-escape.js

@@ -0,0 +1,190 @@
+/**
+ * @fileoverview Disallow useless escape in regex character class
+ * Based on 'no-useless-escape' rule
+ */
+'use strict';
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+const REGEX_CHARCLASS_ESCAPES = new Set('\\bcdDfnrsStvwWxu0123456789]');
+
+/**
+ * Parses a regular expression into a list of regex character class list.
+ * @param {string} regExpText raw text used to create the regular expression
+ * @returns {Object[]} A list of character classes tokens with index and
+ *   escape info
+ * @example
+ *
+ * parseRegExpCharClass('a\\b[cd-]')
+ *
+ * returns:
+ * [
+ *   {
+ *     empty: false,
+ *     start: 4,
+ *     end: 6,
+ *     chars: [
+ *       {text: 'c', index: 4, escaped: false},
+ *       {text: 'd', index: 5, escaped: false},
+ *       {text: '-', index: 6, escaped: false}
+ *     ]
+ *   }
+ * ]
+ */
+
+function parseRegExpCharClass(regExpText) {
+  const charList = [];
+  let charListIdx = -1;
+  const initState = {
+    escapeNextChar: false,
+    inCharClass: false,
+    startingCharClass: false
+  };
+
+  regExpText.split('').reduce((state, char, index) => {
+    if (!state.escapeNextChar) {
+      if (char === '\\') {
+        return Object.assign(state, { escapeNextChar: true });
+      }
+      if (char === '[' && !state.inCharClass) {
+        charListIdx += 1;
+        charList.push({ start: index + 1, chars: [], end: -1 });
+        return Object.assign(state, {
+          inCharClass: true,
+          startingCharClass: true
+        });
+      }
+      if (char === ']' && state.inCharClass) {
+        const charClass = charList[charListIdx];
+        charClass.empty = charClass.chars.length === 0;
+        if (charClass.empty) {
+          charClass.start = charClass.end = -1;
+        } else {
+          charList[charListIdx].end = index - 1;
+        }
+        return Object.assign(state, {
+          inCharClass: false,
+          startingCharClass: false
+        });
+      }
+    }
+    if (state.inCharClass) {
+      charList[charListIdx].chars.push({
+        text: char,
+        index, escaped:
+        state.escapeNextChar
+      });
+    }
+    return Object.assign(state, {
+      escapeNextChar: false,
+      startingCharClass: false
+    });
+  }, initState);
+
+  return charList;
+}
+
+module.exports = {
+  meta: {
+    docs: {
+      description: 'disallow unnecessary regex characer class escape sequences',
+      category: 'Best Practices',
+      recommended: false
+    },
+    fixable: 'code',
+    schema: [{
+      'type': 'object',
+      'properties': {
+        'override': {
+          'type': 'array',
+          'items': { 'type': 'string' },
+          'uniqueItems': true
+        }
+      },
+      'additionalProperties': false
+    }]
+  },
+
+  create(context) {
+    const overrideSet = new Set(context.options.length
+                                  ? context.options[0].override || []
+                                  : []);
+
+    /**
+     * Reports a node
+     * @param {ASTNode} node The node to report
+     * @param {number} startOffset The backslash's offset
+     *   from the start of the node
+     * @param {string} character The uselessly escaped character
+     *   (not including the backslash)
+     * @returns {void}
+     */
+    function report(node, startOffset, character) {
+      context.report({
+        node,
+        loc: {
+          line: node.loc.start.line,
+          column: node.loc.start.column + startOffset
+        },
+        message: 'Unnecessary regex escape in character' +
+                 ' class: \\{{character}}',
+        data: { character },
+        fix: (fixer) => {
+          const start = node.range[0] + startOffset;
+          return fixer.replaceTextRange([start, start + 1], '');
+        }
+      });
+    }
+
+    /**
+     * Checks if a node has superflous escape character
+     *   in regex character class.
+     *
+     * @param {ASTNode} node - node to check.
+     * @returns {void}
+     */
+    function check(node) {
+      if (node.regex) {
+        parseRegExpCharClass(node.regex.pattern)
+          .forEach((charClass) => {
+            charClass
+              .chars
+              // The '-' character is a special case if is not at
+              // either edge of the character class. To account for this,
+              // filter out '-' characters that appear in the middle of a
+              // character class.
+              .filter((charInfo) => !(charInfo.text === '-' &&
+                                      (charInfo.index !== charClass.start &&
+                                       charInfo.index !== charClass.end)))
+
+              // The '^' character is a special case if it's at the beginning
+              // of the character class. To account for this, filter out '^'
+              // characters that appear at the start of a character class.
+              //
+              .filter((charInfo) => !(charInfo.text === '^' &&
+                                    charInfo.index === charClass.start))
+
+              // Filter out characters that aren't escaped.
+              .filter((charInfo) => charInfo.escaped)
+
+              // Filter out characters that are valid to escape, based on
+              // their position in the regular expression.
+              .filter((charInfo) => !REGEX_CHARCLASS_ESCAPES.has(charInfo.text))
+
+              // Filter out overridden character list.
+              .filter((charInfo) => !overrideSet.has(charInfo.text))
+
+              // Report all the remaining characters.
+              .forEach((charInfo) =>
+                report(node, charInfo.index, charInfo.text));
+          });
+      }
+    }
+
+    return {
+      Literal: check
+    };
+  }
+};