Since people are randomly listing various combinations of \\ with other characters, let me for the record repeat here what I wrote in the comments to Double backslashes:

• Every \\ needs to be simplified to \ unless it appears in a \substack or inside a matrix or array-like LaTeX environment (e.g., matrix, align, gather, cases), where it could be legitimately used as a line break. What follows the \\ is otherwise irrelevant, though as a particular case, \\\ or \\\\ should yield \\.

Since people are randomly listing various combinations of \\ with other characters, let me for the record repeat here what I wrote in the comments to Double backslashes:

• Every \\ needs to be simplified to \ unless it appears in a \substack or inside a matrix or array-like LaTeX environment (e.g., matrix, align, gather, cases), where it could be legitimately used as a line break. What follows the \\ is otherwise irrelevant, though as a particular case, \\\ or \\\\ should yield \\.