The slash and the backslash as well are special characters used for glyphtext notation for characters that cannot be rendered directly. In glyphtext notation you can use \uXXXX for a Unicode codepoint or /glyphname to address a glyph by its name.
Please see "Editing a Sample String" section on page 553 of the Mac Manual for details.
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It's on page 541 of the Win Manual. "Slash" and "backslash" are used this way in Preview and also in the Metrics window.