function gsub(source, pattern, replacement) {
	var result = '', match, replaced;
	while (source.length > 0) {
		if (match = source.match(pattern)) {
			result += source.slice(0, match.index);
			replaced = replacement(match);
			result += replaced == null ? '' : String(replaced);
			source = source.slice(match.index + match[0].length);
		} else {
			result += source, source = '';
		}
	}
	return result;
}

Based on Prototype JS String#gsub

JavaScript String gsub

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