When I was writing my linear-gradient() to -webkit-gradient() converter, I knew in advance that I would have to use a quite large regular expression to validate and parse the input. Such a regex would be incredibly hard to read and fix potential issues, so I tried to find a way to cut the process down in reusable parts.

Turns out JavaScript regular expression objects have a .source property that can be used in the RegExp constructor to create a new RegExp out of another one. So I wrote a new function that takes a string with identifiers for regexp replacements in and replaces them with the corresponding sub-regexps, taken from an object literal as a second argument:

/** * Create complex regexps in an easy to read way * @param str {String} Final regex with for replacements * @param replacements {Object} Object with the replacements * @param flags {String} Just like the flags argument in the RegExp constructor */ RegExp.create = function(str, replacements, flags) { for(var id in replacements) { var replacement = replacements[id], idRegExp = RegExp(’ + id + ', ‘gi’);

if(replacement.source) { replacement = replacement.source.replace(/^\^|\$$/g, ‘’); }

// Don’t add extra parentheses if they already exist str = str.replace(RegExp(‘\\(’ + idRegExp.source + ‘\\)’, ‘gi’), ‘(’ + replacement + ‘)’);

str = str.replace(idRegExp, ‘(?:’ + replacement + ‘)’); }

return RegExp(str, flags); };

If you don’t like adding a function to the RegExp object, you can name it however you want. Here’s how I used it for my linear-gradient() parser:

self.regex = {};

self.regex.number = /^-?[0-9]*\.?[0-9]+$/; self.regex.keyword = /^(?:top\s+|bottom\s+)?(?:right|left)|(?:right\s+|left\s+)?(?:top|bottom)$/;

self.regex.direction = RegExp.create(‘^(?:|deg|0)$’, { keyword: self.regex.keyword, number: self.regex.number });

self.regex.color = RegExp.create(‘(?:||)’, { keyword: /^(?:red|tan|grey|gray|lime|navy|blue|teal|aqua|cyan|gold|peru|pink|plum|snow|[a-z]{5,20})$/, func: RegExp.create(‘^(?:rgb|hsl)a?\\((?:\\s*%?\\s*,?\\s*){3,4}\\)$’, { number: self.regex.number }), hex: /^#(?:[0-9a-f]{1,2}){3}$/ });

self.regex.percentage = RegExp.create(‘^(?:%|0)$’, { number: self.regex.number });

self.regex.length = RegExp.create(‘|0’, { number: self.regex.number, unit: /%|px|mm|cm|in|em|rem|en|ex|ch|vm|vw|vh/ });

self.regex.colorStop = RegExp.create(‘\\s*?’, { color: self.regex.color, length: self.regex.length }, ‘g’);

self.regex.linearGradient = RegExp.create(‘^linear-gradient\\(\\s*(?:()\\s*,)?\\s*(\\s*(?:,\\s*\\s*)+)\\)$’, { direction: self.regex.direction, colorStop: self.regex.colorStop }, ‘i’);

(self in this case was a local variable, not the window object)