If you would like to match any digit, write:

String re = "\\d";

Backslashes within string literals in Java source code are interpreted
as required by the Java Language Specification as either Unicode
escapes or other character escapes. It is therefore necessary to double
backslashes in string literals that represent regular expressions to
protect them from interpretation by the Java bytecode compiler. The
string literal "\b", for example, matches a single backspace character
when interpreted as a regular expression, while "\\b" matches a word
boundary. The string literal "\(hello\)" is illegal and leads to a
compile-time error; in order to match the string (hello) the string
literal "\\(hello\\)"  must be used.

If you would like to match the string \d, write

String re = "\\\\d";

Bart

A backslash in a string literal need to be escaped with another
backslash to get it into the string as a single backslash.

/Thomas

see http://mindprod.com/jgloss/regex.html

that should be "\\d";