Java regex escaped characters

Question

When matching certain characters (such as line feed), you can use the regex \"\\\\n\" or indeed just \"\\n\". For example, the following splits a string into an array of lines:<

Answer 1

Yes there are different. The Java Compiler has different behavior for Unicode Escapes in the Java Book The Java Language Specification section 3.3;

The Java programming language specifies a standard way of transforming a program written in Unicode into ASCII that changes a program into a form that can be processed by ASCII-based tools. The transformation involves converting any Unicode escapes in the source text of the program to ASCII by adding an extra u - for example, \uxxxx becomes \uuxxxx - while simultaneously converting non- ASCII characters in the source text to Unicode escapes containing a single u each.

So how this affect the /n vs //n in the Java Doc:

It is therefore necessary to double backslashes in string literals that represent regular expressions to protect them from interpretation by the Java bytecode compiler.

An a example of the same doc:

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.

Answer 2

There is no difference in the current scenario. The usual string escape sequences are formed with the help of a single backslash and then a valid escape char ("\n", "\r", etc.) and regex escape sequences are formed with the help of a literal backslash (that is, a double backslash in the Java string literal) and a valid regex escape char ("\\n", "\\d", etc.).

"\n" (an escape sequence) is a literal LF (newline) and "\\n" is a regex escape sequence that matches an LF symbol.

"\r" (an escape sequence) is a literal CR (carriage return) and "\\r" is a regex escape sequence that matches an CR symbol.

"\t" (an escape sequence) is a literal tab symbol and "\\t" is a regex escape sequence that matches a tab symbol.

See the list in the Java regex docs for the supported list of regex escapes.

However, if you use a Pattern.COMMENTS flag (used to introduce comments and format a pattern nicely, making the regex engine ignore all unescaped whitespace in the pattern), you will need to either use "\\n" or "\\\n" to define a newline (LF) in the Java string literal and "\\r" or "\\\r" to define a carriage return (CR).

See a Java test:

String s = "\n";
System.out.println(s.replaceAll("\n", "LF")); // => LF
System.out.println(s.replaceAll("\\n", "LF")); // => LF
System.out.println(s.replaceAll("(?x)\\n", "LF")); // => LF
System.out.println(s.replaceAll("(?x)\\\n", "LF")); // => LF
System.out.println(s.replaceAll("(?x)\n", "<LF>")); 
// => <LF>
//<LF>

Why is the last one producing <LF>+newline+<LF>? Because "(?x)\n" is equal to "", an empty pattern, and it matches an empty space before the newline and after it.