Backslash is a special character. When you assign

$txt = ' __\\U//__ ';
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, the first backslash is escaping the second, so the string has only one backslash in it. Print it and you'll see. When something other than a backslash or quote character appears after the backslash, the backslash isn't seen as an escape, and is interpreted literally. So your

$txt = ' \*/  ';
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also has one backslash.

Backslash is also special in the replacement side of the s///, so your four become two. To convert one to four, double the number of backslashes on the right.