char-escape-unescape-transform.js

/**
 * The MIT License (MIT)
 * Copyright (c) 2017-present Dmitry Soshnikov <dmitry.soshnikov@gmail.com>
 */

'use strict';

/**
 * A regexp-tree plugin to remove unnecessary escape.
 *
 * \e -> e
 *
 * [\(] -> [(]
 */

module.exports = {
  _hasXFlag: false,
  init: function init(ast) {
    this._hasXFlag = ast.flags.includes('x');
  },
  Char: function Char(path) {
    var node = path.node;


    if (!node.escaped) {
      return;
    }

    if (shouldUnescape(path, this._hasXFlag)) {
      delete node.escaped;
    }
  }
};

function shouldUnescape(path, hasXFlag) {
  var value = path.node.value,
      index = path.index,
      parent = path.parent;

  // In char class (, etc are allowed.

  if (parent.type !== 'CharacterClass' && parent.type !== 'ClassRange') {
    return !preservesEscape(value, index, parent, hasXFlag);
  }

  return !preservesInCharClass(value, index, parent);
}

/**
 * \], \\, \^, \-
 */
function preservesInCharClass(value, index, parent) {
  if (value === '^') {
    // Avoid [\^a] turning into [^a]
    return index === 0 && !parent.negative;
  }
  if (value === '-') {
    // Avoid [a\-z] turning into [a-z]
    return index !== 0 && index !== parent.expressions.length - 1;
  }
  return (/[\]\\]/.test(value)
  );
}

function preservesEscape(value, index, parent, hasXFlag) {
  if (value === '{') {
    return preservesOpeningCurlyBraceEscape(index, parent);
  }

  if (value === '}') {
    return preservesClosingCurlyBraceEscape(index, parent);
  }

  if (hasXFlag && /[ #]/.test(value)) {
    return true;
  }

  return (/[*[()+?^$./\\|]/.test(value)
  );
}

function consumeNumbers(startIndex, parent, rtl) {
  var i = startIndex;
  var siblingNode = (rtl ? i >= 0 : i < parent.expressions.length) && parent.expressions[i];

  while (siblingNode && siblingNode.type === 'Char' && siblingNode.kind === 'simple' && !siblingNode.escaped && /\d/.test(siblingNode.value)) {
    rtl ? i-- : i++;
    siblingNode = (rtl ? i >= 0 : i < parent.expressions.length) && parent.expressions[i];
  }

  return Math.abs(startIndex - i);
}

function isSimpleChar(node, value) {
  return node && node.type === 'Char' && node.kind === 'simple' && !node.escaped && node.value === value;
}

function preservesOpeningCurlyBraceEscape(index, parent) {
  var nbFollowingNumbers = consumeNumbers(index + 1, parent);
  var i = index + nbFollowingNumbers + 1;
  var nextSiblingNode = i < parent.expressions.length && parent.expressions[i];

  if (nbFollowingNumbers) {

    // Avoid \{3} turning into {3}
    if (isSimpleChar(nextSiblingNode, '}')) {
      return true;
    }

    if (isSimpleChar(nextSiblingNode, ',')) {

      nbFollowingNumbers = consumeNumbers(i + 1, parent);
      i = i + nbFollowingNumbers + 1;
      nextSiblingNode = i < parent.expressions.length && parent.expressions[i];

      // Avoid \{3,} turning into {3,}
      return isSimpleChar(nextSiblingNode, '}');
    }
  }
  return false;
}

function preservesClosingCurlyBraceEscape(index, parent) {
  var nbPrecedingNumbers = consumeNumbers(index - 1, parent, true);
  var i = index - nbPrecedingNumbers - 1;
  var previousSiblingNode = i >= 0 && parent.expressions[i];

  // Avoid {3\} turning into {3}
  if (nbPrecedingNumbers && isSimpleChar(previousSiblingNode, '{')) {
    return true;
  }

  if (isSimpleChar(previousSiblingNode, ',')) {

    nbPrecedingNumbers = consumeNumbers(i - 1, parent, true);
    i = i - nbPrecedingNumbers - 1;
    previousSiblingNode = i < parent.expressions.length && parent.expressions[i];

    // Avoid {3,\} turning into {3,}
    return nbPrecedingNumbers && isSimpleChar(previousSiblingNode, '{');
  }
  return false;
}