带反斜杠的输入与正则表达式不匹配

Question

I'm trying to create a regular expression matcher, but it doesn't work as expected.

String input = "// source C:\\path\\to\\folder";
System.out.println(Pattern.matches("//\\s*source\\s+[a-zA-Z]:(\\[a-zA-Z0-9_-]+)+", input));

It returns false but it should pass. What is wrong with that regex?

Answer 1

Backslashes. That's what is wrong.

System.out.println(Pattern.matches("//\\s*source\\s+[a-zA-Z]:(\\\\[a-zA-Z0-9_-]+)+", input));
                                                              ^^

In regex, a backslash must be escaped—backslashed. That's two backslashes. Add to that, Java escaping and you must write four backslashes to match one.

Answer 2

You should use: \\\\\\\\ to match a backslash in Java regex:

String input = "// source C:\\path\\to\\folder";
boolean m = Pattern.matches("//\\s*source\\s+[a-zA-Z]:(\\\\[a-zA-Z0-9_-]+)+", input);
//=> true

You need first escaping ie \\\\ for String and another escaping ie \\\\ for underlying regex engine to get a literal \\ .

Answer 3

You forgot \\\\ in [a-zA-Z0-9_-] :

String input = "// source C:\\path\\to\\folder";
System.out.println(Pattern.matches("//\\s*source\\s+[a-zA-Z]:(\\\\[a-zA-Z0-9_\\-]+)+", input));