Camper-php matches the location of uppercase/lowercase letters in a string

The variable name uses a hump, and the field in the database contains underscores. Now I want to convert userId and other variables into user_id in batches. How can I get the position of uppercase letters in the string? The variable name uses a hump, and the field in the database contains underscores. Now I want to convert userId and other variables into user_id in batches. How can I get the position of uppercase letters in the string?

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The variable name uses a hump, and the field in the database contains underscores. Now I want to convert userId and other variables into user_id in batches. How can I get the position of uppercase letters in the string?

Http://3v4l.org/kC52c

echo preg_replace_callback("/[A-Z]/", function($ma){return "_".strtolower($ma[0]);}, "userId"); //user_id

Thank you @ brief for your answer. I found another practical website.

The answer to explainOpenZIPChangeopen_z_i_p.

The following is my code:

Http://3v4l.org/hIQUW

echo strtolower(preg_replace('/((?<=[a-z])(?=[A-Z]))/', '_', 'AbcDefGhijk'));

Try it.OpenZIPChangedopen_zip.

From:Laravel4

     /**     * Convert a string to snake case.     *     * @param  string  $value     * @param  string  $delimiter     * @return string     */    function snake($value, $delimiter = '_')    {        $replace = '$1'.$delimiter.'$2';        return ctype_lower($value) ? $value : strtolower(preg_replace('/(.)([A-Z])/', $replace, $value));    }

Echo preg_replace ('/([A-Z])/', '_ $ 1', 'abcdefghijk'); it seems easier