Why four backslashes in the regular expression?

Question

"^\\\\d{1,2}$"

I have the above regex. I know that the string parser will remove two backlashes leaving us with \\d. Taking one for the meta-character, what is the function of the extraneous \ ? I haven't had previous experience in regex. Or is the string pattern is in itself [backslash][integer up to two occurences]. Am I missing something ?

Answer 1

Backslashes escape other backslashes, as well as special characters.

What you have there is:

• \d is "digit", in your regex engine.
• \\d is backslash-escaping-backslash + d, == \d, in your string quoting mechanism.
• \\\\d is backslash-escaping-backslash, twice, +d, probably escaping the command line if you're using a shell, or if you have to pass the string through system or rsh or something.