Matching a backslash

If you want to match a backslash in your regular expression, you’ll have to escape it.

Backslash is an escape character in regular expressions. You can use ‘\\’ to refer to a single backslash in a regular expression.

However, backslash is also an escape character in Java literal strings. To make a regular expression from a string literal, you have to escape each of its backslashes. In a string literal ‘\\\\’ can be used to create a regular expression with ‘\\’, which in turn can match ‘\’.

For example, consider matching strings like “C:\dir\myfile.txt”. A regular expression ([A-Za-z]):\\(.*) will match, and provide the drive letter as a capturing group. Note the doubled backslash.

To express that pattern in a Java string literal, each of the backslashes in the regular expression needs to be escaped.

String path = "C:\\dir\\myfile.txt";
System.out.println( "Local path: " + path ); // "C:\dir\myfile.txt"

String regex = "([A-Za-z]):\\\\.*"; // Four to match one
System.out.println("Regex:      " + regex ); // "([A-Za-z]):\\(.*)"

Pattern pattern = Pattern.compile( regex );
Matcher matcher = pattern.matcher( path );
if ( matcher.matches()) {
    System.out.println( "This path is on drive " + matcher.group( 1 ) + ":.");
    // This path is on drive C:.
}

If you want to match two backslashes, you’ll find yourself using eight in a literal string, to represent four in the regular expression, to match two.

String path = "\\\\myhost\\share\\myfile.txt";
System.out.println( "UNC path: " + path ); // \\myhost\share\myfile.txt"

String regex = "\\\\\\\\(.+?)\\\\(.*)"; // Eight to match two
System.out.println("Regex:    " + regex ); // \\\\(.+?)\\(.*) 

Pattern pattern = Pattern.compile( regex );
Matcher matcher = pattern.matcher( path );

if ( matcher.matches()) {
    System.out.println( "This path is on host '" + matcher.group( 1 ) + "'.");
    // This path is on host 'myhost'.
}