Explain why each of these strings don't match a \: "\", "\\", "\\\"

"\": This will escape the next character in the R string.
"\": This will resolve to \ in the regular expression, which will escape the next character in the regular expression.
"\\": The first two backslashes will resolve to a literal backslash in the regular expression, the third will escape the next character. So in the regular expression, this will escape some escaped character.