Can't escape the backslash with regex?

If you're putting this in a string within a program, you may actually need to use four backslashes (because the string parser will remove two of them when "de-escaping" it for the string, and then the regex needs two for an escaped regex backslash).

For instance:

regex("\\\\")

is interpreted as...

regex("\\" [escaped backslash] followed by "\\" [escaped backslash])

is interpreted as...

regex(\\)

is interpreted as a regex that matches a single backslash.

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Depending on the language, you might be able to use a different form of quoting that doesn't parse escape sequences to avoid having to use as many - for instance, in Python:

re.compile(r'\\')

The r in front of the quotes makes it a raw string which doesn't parse backslash escapes.

If it's not a literal, you have to use \\\\ so that you get \\ which means an escaped backslash.

That's because there are two representations. In the string representation of your regex, you have "\\\\", Which is what gets sent to the parser. The parser will see \\ which it interprets as a valid escaped-backslash (which matches a single backslash).

The backslash \ is the escape character for regular expressions. Therefore a double backslash would indeed mean a single, literal backslash.

\ (backslash) followed by any of [\^$.|?*+(){} escapes the special character to suppress its special meaning.

ref : http://www.regular-expressions.info/reference.html