index.js

var plugin = function (options) {
  options = options || {};
  options.preserveBeforeAfter = options.preserveBeforeAfter || true;

  // Backwards compatibility--we always by default ignored `:root`.
  var blacklist = {
    ':root': true,
    ':host': true,
    ':host-context': true
  };

  var prefix = options.prefix || '\\:';

  (options.blacklist || []).forEach(function (blacklistItem) {
    blacklist[blacklistItem] = true;
  });

  var restrictTo;

  if (Array.isArray(options.restrictTo) && options.restrictTo.length) {
    restrictTo = options.restrictTo.reduce(function (target, pseudoClass) {
      var finalClass =
        (pseudoClass.charAt(0) === ':' ? '' : ':') +
        pseudoClass.replace(/\(.*/g, '');
      if (!Object.prototype.hasOwnProperty.call(target, finalClass)) {
        target[finalClass] = true;
      }
      return target;
    }, {});
  }

  return {
    postcssPlugin: 'postcss-pseudo-classes',
    prepare: function () {
      var fixed = [];
      return {
        Rule: function (rule) {
          if (fixed.indexOf(rule) !== -1) {
            return;
          }
          fixed.push(rule);

          var combinations;

          rule.selectors.forEach(function (selector) {
            // Ignore some popular things that are never useful
            if (blacklist[selector]) {
              return;
            }

            if (!selector.includes(':')) {
              return;
            }

            var selectorParts = selector.split(' ');
            var pseudoedSelectorParts = [];

            selectorParts.forEach(function (selectorPart, index) {
              var pseudos = selectorPart.match(/::?([^:]+)/g);

              if (!pseudos) {
                if (options.allCombinations) {
                  pseudoedSelectorParts[index] = [selectorPart];
                } else {
                  pseudoedSelectorParts.push(selectorPart);
                }
                return;
              }

              var baseSelector = selectorPart.substr(
                0,
                selectorPart.length - pseudos.join('').length
              );

              var classPseudos = pseudos.map(function (pseudo) {
                var pseudoToCheck = pseudo.replace(/\(.*/g, '');
                // restrictTo a subset of pseudo classes
                if (
                  blacklist[pseudoToCheck] ||
                  (restrictTo && !restrictTo[pseudoToCheck])
                ) {
                  return pseudo;
                }

                // Ignore pseudo-elements!
                if (pseudo.match(/^::/)) {
                  return pseudo;
                }

                // Ignore ':before' and ':after'
                if (
                  options.preserveBeforeAfter &&
                  [':before', ':after'].indexOf(pseudo) !== -1
                ) {
                  return pseudo;
                }

                // Kill the colon
                pseudo = pseudo.substr(1);

                // Replace left and right parens
                pseudo = pseudo.replace(/\(/g, '\\(');
                pseudo = pseudo.replace(/\)/g, '\\)');

                return '.' + prefix + pseudo;
              });

              // Add all combinations of pseudo selectors/pseudo styles given a
              // selector with multiple pseudo styles.
              if (options.allCombinations) {
                combinations = createCombinations(pseudos, classPseudos);
                pseudoedSelectorParts[index] = [];

                combinations.forEach(function (combination) {
                  pseudoedSelectorParts[index].push(baseSelector + combination);
                });
              } else {
                pseudoedSelectorParts.push(
                  baseSelector + classPseudos.join('')
                );
              }
            });

            if (options.allCombinations) {
              var serialCombinations = createSerialCombinations(
                pseudoedSelectorParts,
                appendWithSpace
              );

              serialCombinations.forEach(function (combination) {
                addSelector(combination);
              });
            } else {
              addSelector(pseudoedSelectorParts.join(' '));
            }

            function addSelector(newSelector) {
              if (newSelector && newSelector !== selector) {
                rule.selector += ',\n' + newSelector;
              }
            }
          });
        }
      };
    }
  };
};
plugin.postcss = true;

// a.length === b.length
function createCombinations(a, b) {
  var combinations = [''];
  var newCombinations;
  for (var i = 0, len = a.length; i < len; i += 1) {
    newCombinations = [];
    combinations.forEach(function (combination) {
      newCombinations.push(combination + a[i]);
      // Don't repeat work.
      if (a[i] !== b[i]) {
        newCombinations.push(combination + b[i]);
      }
    });
    combinations = newCombinations;
  }
  return combinations;
}

// arr = [[list of 1st el], [list of 2nd el] ... etc]
function createSerialCombinations(arr, fn) {
  var combinations = [''];
  var newCombinations;
  arr.forEach(function (elements) {
    newCombinations = [];
    elements.forEach(function (element) {
      combinations.forEach(function (combination) {
        newCombinations.push(fn(combination, element));
      });
    });
    combinations = newCombinations;
  });
  return combinations;
}

function appendWithSpace(a, b) {
  if (a) {
    a += ' ';
  }
  return a + b;
}

module.exports = plugin;