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<?php function camelCaseToUnderscore($input) { return ltrim(strtolower(preg_replace('/[A-Z]([A-Z](?![a-z]))*/', '_$0', $input)), '_'); } echo camelCaseToUnderscore('CamelCaseForTheWin'), "\n"; echo camelCaseToUnderscore('ACar'), "\n"; echo camelCaseToUnderscore('BuyACar'), "\n"; echo camelCaseToUnderscore('XML'), "\n"; echo camelCaseToUnderscore('SimpleXML'), "\n";